[PATCH v3 6/7] sched: Implement shared runqueue in CFS

From: David Vernet
Date: Wed Aug 09 2023 - 18:13:16 EST

Overview
========

The scheduler must constantly strike a balance between work
conservation, and avoiding costly migrations which harm performance due
to e.g. decreased cache locality. The matter is further complicated by
the topology of the system. Migrating a task between cores on the same
LLC may be more optimal than keeping a task local to the CPU, whereas
migrating a task between LLCs or NUMA nodes may tip the balance in the
other direction.

With that in mind, while CFS is by and large mostly a work conserving
scheduler, there are certain instances where the scheduler will choose
to keep a task local to a CPU, when it would have been more optimal to
migrate it to an idle core.

An example of such a workload is the HHVM / web workload at Meta. HHVM
is a VM that JITs Hack and PHP code in service of web requests. Like
other JIT / compilation workloads, it tends to be heavily CPU bound, and
exhibit generally poor cache locality. To try and address this, we set
several debugfs (/sys/kernel/debug/sched) knobs on our HHVM workloads:

- migration_cost_ns -> 0
- latency_ns -> 20000000
- min_granularity_ns -> 10000000
- wakeup_granularity_ns -> 12000000

These knobs are intended both to encourage the scheduler to be as work
conserving as possible (migration_cost_ns -> 0), and also to keep tasks
running for relatively long time slices so as to avoid the overhead of
context switching (the other knobs). Collectively, these knobs provide a
substantial performance win; resulting in roughly a 20% improvement in
throughput. Worth noting, however, is that this improvement is _not_ at
full machine saturation.

That said, even with these knobs, we noticed that CPUs were still going
idle even when the host was overcommitted. In response, we wrote the
"shared runqueue" (SHARED_RUNQ) feature proposed in this patch set. The
idea behind SHARED_RUNQ is simple: it enables the scheduler to be more
aggressively work conserving by placing a waking task into a sharded
per-LLC FIFO queue that can be pulled from by another core in the LLC
FIFO queue which can then be pulled from before it goes idle.

With this simple change, we were able to achieve a 1 - 1.6% improvement
in throughput, as well as a small, consistent improvement in p95 and p99
latencies, in HHVM. These performance improvements were in addition to
the wins from the debugfs knobs mentioned above, and to other benchmarks
outlined below in the Results section.

Design
======

Note that the design described here reflects sharding, which will be
added in a subsequent patch. The design is described that way in this
commit summary as the benchmarks described in the results section below
all include sharded SHARED_RUNQ. The patches are not combined into one
to ease the burden of review.

The design of SHARED_RUNQ is quite simple. A shared_runq is simply a
list of struct shared_runq_shard objects, which itself is simply a
struct list_head of tasks, and a spinlock:

struct shared_runq_shard {
struct list_head list;
raw_spinlock_t lock;
} ____cacheline_aligned;

struct shared_runq {
u32 num_shards;
struct shared_runq_shard shards[];
} ____cacheline_aligned;

We create a struct shared_runq per LLC, ensuring they're in their own
cachelines to avoid false sharing between CPUs on different LLCs, and we
create a number of struct shared_runq_shard objects that are housed
there.

When a task first wakes up, it enqueues itself in the shared_runq_shard
of its current LLC at the end of enqueue_task_fair(). Enqueues only
happen if the task was not manually migrated to the current core by
select_task_rq(), and is not pinned to a specific CPU.

A core will pull a task from the shards in its LLC's shared_runq at the
beginning of newidle_balance().

Difference between SHARED_RUNQ and SIS_NODE
===========================================

In [0] Peter proposed a patch that addresses Tejun's observations that
when workqueues are targeted towards a specific LLC on his Zen2 machine
with small CCXs, that there would be significant idle time due to
select_idle_sibling() not considering anything outside of the current
LLC.

This patch (SIS_NODE) is essentially the complement to the proposal
here. SID_NODE causes waking tasks to look for idle cores in neighboring
LLCs on the same die, whereas SHARED_RUNQ causes cores about to go idle
to look for enqueued tasks. That said, in its current form, the two
features at are a different scope as SIS_NODE searches for idle cores
between LLCs, while SHARED_RUNQ enqueues tasks within a single LLC.

The patch was since removed in [1], and we compared the results to
shared_Runq (previously called "swqueue") in [2]. SIS_NODE did not
outperform SHARED_RUNQ on any of the benchmarks, so we elect to not
compare against it again for this v2 patch set.

[0]: https://lore.kernel.org/all/20230530113249.GA156198@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[1]: https://lore.kernel.org/all/20230605175636.GA4253@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[2]: https://lore.kernel.org/lkml/20230613052004.2836135-1-void@xxxxxxxxxxxxx/

Worth noting as well is that pointed out in [3] that the logic behind
including SIS_NODE in the first place should apply to SHARED_RUNQ
(meaning that e.g. very small Zen2 CPUs with only 3/4 cores per LLC
should benefit from having a single shared_runq stretch across multiple
LLCs). I drafted a patch that implements this by having a minimum LLC
size for creating a shard, and stretches a shared_runq across multiple
LLCs if they're smaller than that size, and sent it to Tejun to test on
his Zen2. Tejun reported back that SIS_NODE did not seem to make a
difference:

[3]: https://lore.kernel.org/lkml/20230711114207.GK3062772@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

o____________o__________o
| mean | Variance |
o------------o----------o
Vanilla: | 108.84s | 0.0057 |
NO_SHARED_RUNQ: | 108.82s | 0.119s |
SHARED_RUNQ: | 108.17s | 0.038s |
SHARED_RUNQ w/ SIS_NODE: | 108.87s | 0.111s |
o------------o----------o

I similarly tried running kcompile on SHARED_RUNQ with SIS_NODE on my
7950X Zen3, but didn't see any gain relative to plain SHARED_RUNQ (though
a gain was observed relative to NO_SHARED_RUNQ, as described below).

Results
=======

Note that the motivation for the shared runqueue feature was originally
arrived at using experiments in the sched_ext framework that's currently
being proposed upstream. The ~1 - 1.6% improvement in HHVM throughput
is similarly visible using work-conserving sched_ext schedulers (even
very simple ones like global FIFO).

In both single and multi socket / CCX hosts, this can measurably improve
performance. In addition to the performance gains observed on our
internal web workloads, we also observed an improvement in common
workloads such as kernel compile and hackbench, when running shared
runqueue.

On the other hand, some workloads suffer from SHARED_RUNQ. Workloads
that hammer the runqueue hard, such as netperf UDP_RR, or schbench -L
-m 52 -p 512 -r 10 -t 1. This can be mitigated somewhat by sharding the
shared datastructures within a CCX, but it doesn't seem to eliminate all
contention in every scenario. On the positive side, it seems that
sharding does not materially harm the benchmarks run for this patch
series; and in fact seems to improve some workloads such as kernel
compile.

Note that for the kernel compile workloads below, the compilation was
done by running make -j$(nproc) built-in.a on several different types of
hosts configured with make allyesconfig on commit a27648c74210 ("afs:
Fix setting of mtime when creating a file/dir/symlink") on Linus' tree
(boost and turbo were disabled on all of these hosts when the
experiments were performed).

Finally, note that these results were from the patch set built off of
commit ebb83d84e49b ("sched/core: Avoid multiple calling
update_rq_clock() in __cfsb_csd_unthrottle()") on the sched/core branch
of tip.git for easy comparison with the v2 patch set results. The
patches in their final form from this set were rebased onto commit
88c56cfeaec4 ("sched/fair: Block nohz tick_stop when cfs bandwidth in
use").

=== Single-socket | 16 core / 32 thread | 2-CCX | AMD 7950X Zen4 ===

CPU max MHz: 5879.8818
CPU min MHz: 3000.0000

Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 581.95s | 2.639s |
SHARED_RUNQ: | 577.02s | 0.084s |
o------------o----------o

Takeaway: SHARED_RUNQ results in a statistically significant ~.85%
improvement over NO_SHARED_RUNQ. This suggests that enqueuing tasks in
the shared runqueue on every enqueue improves work conservation, and
thanks to sharding, does not result in contention.

Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 2.2492s | .00001s |
SHARED_RUNQ: | 2.0217s | .00065s |
o------------o----------o

Takeaway: SHARED_RUNQ in both forms performs exceptionally well compared
to NO_SHARED_RUNQ here, beating it by over 10%. This was a surprising
result given that it seems advantageous to err on the side of avoiding
migration in hackbench given that tasks are short lived in sending only
10k bytes worth of messages, but the results of the benchmark would seem
to suggest that minimizing runqueue delays is preferable.

Command:
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 25037.45 | 2243.44 |
SHARED_RUNQ: | 24952.50 | 1268.06 |
o-----------------------o

Takeaway: No statistical significance, though it is worth noting that
there is no regression for shared runqueue on the 7950X, while there is
a small regression on the Skylake and Milan hosts for SHARED_RUNQ as
described below.

=== Single-socket | 18 core / 36 thread | 1-CCX | Intel Skylake ===

CPU max MHz: 1601.0000
CPU min MHz: 800.0000

Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 1517.44s | 2.8322s |
SHARED_RUNQ: | 1516.51s | 2.9450s |
o------------o----------o

Takeaway: There's on statistically significant gain here. I observed
what I claimed was a .23% win in v2, but it appears that this is not
actually statistically significant.

Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 5.3370s | .0012s |
SHARED_RUNQ: | 5.2668s | .0033s |
o------------o----------o

Takeaway: SHARED_RUNQ results in a ~1.3% improvement over
NO_SHARED_RUNQ. Also statistically significant, but smaller than the
10+% improvement observed on the 7950X.

Command: netperf -n $(nproc) -l 60 -t TCP_RR
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 15699.32 | 377.01 |
SHARED_RUNQ: | 14966.42 | 714.13 |
o-----------------------o

Takeaway: NO_SHARED_RUNQ beats SHARED_RUNQ by ~4.6%. This result makes
sense -- the workload is very heavy on the runqueue, so enqueuing tasks
in the shared runqueue in __enqueue_entity() would intuitively result in
increased contention on the shard lock.

=== Single-socket | 72-core | 6-CCX | AMD Milan Zen3 ===

CPU max MHz: 700.0000
CPU min MHz: 700.0000

Command: make -j$(nproc) built-in.a
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 1568.55s | 0.1568s |
SHARED_RUNQ: | 1568.26s | 1.2168s |
o------------o----------o

Takeaway: No statistically significant difference here. It might be
worth experimenting with work stealing in a follow-on patch set.

Command: hackbench --loops 10000
o____________o__________o
| mean | Variance |
o------------o----------o
NO_SHARED_RUNQ: | 5.2716s | .00143s |
SHARED_RUNQ: | 5.1716s | .00289s |
o------------o----------o

Takeaway: SHARED_RUNQ again wins, by about 2%.

Command: netperf -n $(nproc) -l 60 -t TCP_RR
for i in `seq 128`; do
netperf -6 -t UDP_RR -c -C -l $runtime &
done
o_______________________o
| Throughput | Variance |
o-----------------------o
NO_SHARED_RUNQ: | 17482.03 | 4675.99 |
SHARED_RUNQ: | 16697.25 | 9812.23 |
o-----------------------o

Takeaway: Similar to the Skylake runs, NO_SHARED_RUNQ still beats
SHARED_RUNQ, this time by ~4.5%. It's worth noting that in v2, the
NO_SHARED_RUNQ was only ~1.8% faster. The variance is very high here, so
the results of this benchmark should be taken with a large grain of
salt (noting that we do consistently see NO_SHARED_RUNQ on top due to
not contending on the shard lock).

Finally, let's look at how sharding affects the following schbench
incantation suggested by Chris in [4]:

schbench -L -m 52 -p 512 -r 10 -t 1

[4]: https://lore.kernel.org/lkml/c8419d9b-2b31-2190-3058-3625bdbcb13d@xxxxxxxx/

The TL;DR is that sharding improves things a lot, but doesn't completely
fix the problem. Here are the results from running the schbench command
on the 18 core / 36 thread single CCX, single-socket Skylake:

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

&shard->lock: 31510503 31510711 0.08 19.98 168932319.64 5.36 31700383 31843851 0.03 17.50 10273968.33 0.32
------------
&shard->lock 15731657 [<0000000068c0fd75>] pick_next_task_fair+0x4dd/0x510
&shard->lock 15756516 [<000000001faf84f9>] enqueue_task_fair+0x459/0x530
&shard->lock 21766 [<00000000126ec6ab>] newidle_balance+0x45a/0x650
&shard->lock 772 [<000000002886c365>] dequeue_task_fair+0x4c9/0x540
------------
&shard->lock 23458 [<00000000126ec6ab>] newidle_balance+0x45a/0x650
&shard->lock 16505108 [<000000001faf84f9>] enqueue_task_fair+0x459/0x530
&shard->lock 14981310 [<0000000068c0fd75>] pick_next_task_fair+0x4dd/0x510
&shard->lock 835 [<000000002886c365>] dequeue_task_fair+0x4c9/0x540

These results are when we create only 3 shards (16 logical cores per
shard), so the contention may be a result of overly-coarse sharding. If
we run the schbench incantation with no sharding whatsoever, we see the
following significantly worse lock stats contention:

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

&shard->lock: 117868635 118361486 0.09 393.01 1250954097.25 10.57 119345882 119780601 0.05 343.35 38313419.51 0.32
------------
&shard->lock 59169196 [<0000000060507011>] __enqueue_entity+0xdc/0x110
&shard->lock 59084239 [<00000000f1c67316>] __dequeue_entity+0x78/0xa0
&shard->lock 108051 [<00000000084a6193>] newidle_balance+0x45a/0x650
------------
&shard->lock 60028355 [<0000000060507011>] __enqueue_entity+0xdc/0x110
&shard->lock 119882 [<00000000084a6193>] newidle_balance+0x45a/0x650
&shard->lock 58213249 [<00000000f1c67316>] __dequeue_entity+0x78/0xa0

The contention is ~3-4x worse if we don't shard at all. This roughly
matches the fact that we had 3 shards on the first workload run above.
If we make the shards even smaller, the contention is comparably much
lower:

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

&shard->lock: 13839849 13877596 0.08 13.23 5389564.95 0.39 46910241 48069307 0.06 16.40 16534469.35 0.34
------------
&shard->lock 3559 [<00000000ea455dcc>] newidle_balance+0x45a/0x650
&shard->lock 6992418 [<000000002266f400>] __dequeue_entity+0x78/0xa0
&shard->lock 6881619 [<000000002a62f2e0>] __enqueue_entity+0xdc/0x110
------------
&shard->lock 6640140 [<000000002266f400>] __dequeue_entity+0x78/0xa0
&shard->lock 3523 [<00000000ea455dcc>] newidle_balance+0x45a/0x650
&shard->lock 7233933 [<000000002a62f2e0>] __enqueue_entity+0xdc/0x110

Interestingly, SHARED_RUNQ performs worse than NO_SHARED_RUNQ on the schbench
benchmark on Milan as well, but we contend more on the rq lock than the
shard lock:

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

&rq->__lock: 9617614 9656091 0.10 79.64 69665812.00 7.21 18092700 67652829 0.11 82.38 344524858.87 5.09
-----------
&rq->__lock 6301611 [<000000003e63bf26>] task_rq_lock+0x43/0xe0
&rq->__lock 2530807 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 109360 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 178218 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
-----------
&rq->__lock 3245506 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 1294355 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
&rq->__lock 2837804 [<000000003e63bf26>] task_rq_lock+0x43/0xe0
&rq->__lock 1627866 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10

..................................................................................................................................................................................................

&shard->lock: 7338558 7343244 0.10 35.97 7173949.14 0.98 30200858 32679623 0.08 35.59 16270584.52 0.50
------------
&shard->lock 2004142 [<00000000f8aa2c91>] __dequeue_entity+0x78/0xa0
&shard->lock 2611264 [<00000000473978cc>] newidle_balance+0x45a/0x650
&shard->lock 2727838 [<0000000028f55bb5>] __enqueue_entity+0xdc/0x110
------------
&shard->lock 2737232 [<00000000473978cc>] newidle_balance+0x45a/0x650
&shard->lock 1693341 [<00000000f8aa2c91>] __dequeue_entity+0x78/0xa0
&shard->lock 2912671 [<0000000028f55bb5>] __enqueue_entity+0xdc/0x110

...................................................................................................................................................................................................

If we look at the lock stats with SHARED_RUNQ disabled, the rq lock still
contends the most, but it's significantly less than with it enabled:

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

&rq->__lock: 791277 791690 0.12 110.54 4889787.63 6.18 1575996 62390275 0.13 112.66 316262440.56 5.07
-----------
&rq->__lock 263343 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 19394 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 4143 [<000000003b542e83>] __task_rq_lock+0x51/0xf0
&rq->__lock 51094 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170
-----------
&rq->__lock 23756 [<0000000011be1562>] raw_spin_rq_lock_nested+0xa/0x10
&rq->__lock 379048 [<00000000516703f0>] __schedule+0x72/0xaa0
&rq->__lock 677 [<000000003b542e83>] __task_rq_lock+0x51/0xf0

Worth noting is that increasing the granularity of the shards in general
improves very runqueue-heavy workloads such as netperf UDP_RR and this
schbench command, but it doesn't necessarily make a big difference for
every workload, or for sufficiently small CCXs such as the 7950X. It may
make sense to eventually allow users to control this with a debugfs
knob, but for now we'll elect to choose a default that resulted in good
performance for the benchmarks run for this patch series.

Conclusion
==========

SHARED_RUNQ in this form provides statistically significant wins for
several types of workloads, and various CPU topologies. The reason for
this is roughly the same for all workloads: SHARED_RUNQ encourages work
conservation inside of a CCX by having a CPU do an O(# per-LLC shards)
iteration over the shared_runq shards in an LLC. We could similarly do
an O(n) iteration over all of the runqueues in the current LLC when a
core is going idle, but that's quite costly (especially for larger
LLCs), and sharded SHARED_RUNQ seems to provide a performant middle
ground between doing an O(n) walk, and doing an O(1) pull from a single
per-LLC shared runq.

For the workloads above, kernel compile and hackbench were clear winners
for SHARED_RUNQ (especially in __enqueue_entity()). The reason for the
improvement in kernel compile is of course that we have a heavily
CPU-bound workload where cache locality doesn't mean much; getting a CPU
is the #1 goal. As mentioned above, while I didn't expect to see an
improvement in hackbench, the results of the benchmark suggest that
minimizing runqueue delays is preferable to optimizing for L1/L2
locality.

Not all workloads benefit from SHARED_RUNQ, however. Workloads that
hammer the runqueue hard, such as netperf UDP_RR, or schbench -L -m 52
-p 512 -r 10 -t 1, tend to run into contention on the shard locks;
especially when enqueuing tasks in __enqueue_entity(). This can be
mitigated significantly by sharding the shared datastructures within a
CCX, but it doesn't eliminate all contention, as described above.

Finally, while SHARED_RUNQ in this form encourages work conservation, it
of course does not guarantee it given that we don't implement any kind
of work stealing between shared_runq's. In the future, we could
potentially push CPU utilization even higher by enabling work stealing
between shared_runq's, likely between CCXs on the same NUMA node.

Originally-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
---
include/linux/sched.h | 2 +
kernel/sched/core.c | 13 +++
kernel/sched/fair.c | 238 +++++++++++++++++++++++++++++++++++++++-
kernel/sched/sched.h | 4 +
kernel/sched/topology.c | 4 +-
5 files changed, 256 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2aab7be46f7e..8238069fd852 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -769,6 +769,8 @@ struct task_struct {
unsigned long wakee_flip_decay_ts;
struct task_struct *last_wakee;

+ struct list_head shared_runq_node;
+
/*
* recent_used_cpu is initially set as the last CPU used by a task
* that wakes affine another task. Waker/wakee relationships can
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 385c565da87f..fb7e71d3dc0a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4529,6 +4529,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
#ifdef CONFIG_SMP
p->wake_entry.u_flags = CSD_TYPE_TTWU;
p->migration_pending = NULL;
+ INIT_LIST_HEAD(&p->shared_runq_node);
#endif
init_sched_mm_cid(p);
}
@@ -9764,6 +9765,18 @@ int sched_cpu_deactivate(unsigned int cpu)
return 0;
}

+void sched_update_domains(void)
+{
+ const struct sched_class *class;
+
+ update_sched_domain_debugfs();
+
+ for_each_class(class) {
+ if (class->update_domains)
+ class->update_domains();
+ }
+}
+
static void sched_rq_cpu_starting(unsigned int cpu)
{
struct rq *rq = cpu_rq(cpu);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9c23e3b948fc..6e740f8da578 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -139,20 +139,235 @@ static int __init setup_sched_thermal_decay_shift(char *str)
}
__setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);

+/**
+ * struct shared_runq - Per-LLC queue structure for enqueuing and migrating
+ * runnable tasks within an LLC.
+ *
+ * WHAT
+ * ====
+ *
+ * This structure enables the scheduler to be more aggressively work
+ * conserving, by placing waking tasks on a per-LLC FIFO queue that can then be
+ * pulled from when another core in the LLC is going to go idle.
+ *
+ * struct rq stores a pointer to its LLC's shared_runq via struct cfs_rq.
+ * Waking tasks are enqueued in the calling CPU's struct shared_runq in
+ * __enqueue_entity(), and are opportunistically pulled from the shared_runq
+ * in newidle_balance(). Tasks enqueued in a shared_runq may be scheduled prior
+ * to being pulled from the shared_runq, in which case they're simply dequeued
+ * from the shared_runq in __dequeue_entity().
+ *
+ * There is currently no task-stealing between shared_runqs in different LLCs,
+ * which means that shared_runq is not fully work conserving. This could be
+ * added at a later time, with tasks likely only being stolen across
+ * shared_runqs on the same NUMA node to avoid violating NUMA affinities.
+ *
+ * HOW
+ * ===
+ *
+ * A shared_runq is comprised of a list, and a spinlock for synchronization.
+ * Given that the critical section for a shared_runq is typically a fast list
+ * operation, and that the shared_runq is localized to a single LLC, the
+ * spinlock will typically only be contended on workloads that do little else
+ * other than hammer the runqueue.
+ *
+ * WHY
+ * ===
+ *
+ * As mentioned above, the main benefit of shared_runq is that it enables more
+ * aggressive work conservation in the scheduler. This can benefit workloads
+ * that benefit more from CPU utilization than from L1/L2 cache locality.
+ *
+ * shared_runqs are segmented across LLCs both to avoid contention on the
+ * shared_runq spinlock by minimizing the number of CPUs that could contend on
+ * it, as well as to strike a balance between work conservation, and L3 cache
+ * locality.
+ */
+struct shared_runq {
+ struct list_head list;
+ raw_spinlock_t lock;
+} ____cacheline_aligned;
+
#ifdef CONFIG_SMP
+
+static DEFINE_PER_CPU(struct shared_runq, shared_runqs);
+
+static struct shared_runq *rq_shared_runq(struct rq *rq)
+{
+ return rq->cfs.shared_runq;
+}
+
+static void shared_runq_reassign_domains(void)
+{
+ int i;
+ struct shared_runq *shared_runq;
+ struct rq *rq;
+ struct rq_flags rf;
+
+ for_each_possible_cpu(i) {
+ rq = cpu_rq(i);
+ shared_runq = &per_cpu(shared_runqs, per_cpu(sd_llc_id, i));
+
+ rq_lock(rq, &rf);
+ rq->cfs.shared_runq = shared_runq;
+ rq_unlock(rq, &rf);
+ }
+}
+
+static void __shared_runq_drain(struct shared_runq *shared_runq)
+{
+ struct task_struct *p, *tmp;
+
+ raw_spin_lock(&shared_runq->lock);
+ list_for_each_entry_safe(p, tmp, &shared_runq->list, shared_runq_node)
+ list_del_init(&p->shared_runq_node);
+ raw_spin_unlock(&shared_runq->lock);
+}
+
+static void update_domains_fair(void)
+{
+ int i;
+ struct shared_runq *shared_runq;
+
+ /* Avoid racing with SHARED_RUNQ enable / disable. */
+ lockdep_assert_cpus_held();
+
+ shared_runq_reassign_domains();
+
+ /* Ensure every core sees its updated shared_runq pointers. */
+ synchronize_rcu();
+
+ /*
+ * Drain all tasks from all shared_runq's to ensure there are no stale
+ * tasks in any prior domain runq. This can cause us to drain live
+ * tasks that would otherwise have been safe to schedule, but this
+ * isn't a practical problem given how infrequently domains are
+ * rebuilt.
+ */
+ for_each_possible_cpu(i) {
+ shared_runq = &per_cpu(shared_runqs, i);
+ __shared_runq_drain(shared_runq);
+ }
+}
+
void shared_runq_toggle(bool enabling)
-{}
+{
+ int cpu;
+
+ if (enabling)
+ return;
+
+ /* Avoid racing with hotplug. */
+ lockdep_assert_cpus_held();
+
+ /* Ensure all cores have stopped enqueueing / dequeuing tasks. */
+ synchronize_rcu();
+
+ for_each_possible_cpu(cpu) {
+ int sd_id;
+
+ sd_id = per_cpu(sd_llc_id, cpu);
+ if (cpu == sd_id)
+ __shared_runq_drain(rq_shared_runq(cpu_rq(cpu)));
+ }
+}
+
+static struct task_struct *shared_runq_pop_task(struct rq *rq)
+{
+ struct task_struct *p;
+ struct shared_runq *shared_runq;
+
+ shared_runq = rq_shared_runq(rq);
+ if (list_empty(&shared_runq->list))
+ return NULL;
+
+ raw_spin_lock(&shared_runq->lock);
+ p = list_first_entry_or_null(&shared_runq->list, struct task_struct,
+ shared_runq_node);
+ if (p && is_cpu_allowed(p, cpu_of(rq)))
+ list_del_init(&p->shared_runq_node);
+ else
+ p = NULL;
+ raw_spin_unlock(&shared_runq->lock);
+
+ return p;
+}
+
+static void shared_runq_push_task(struct rq *rq, struct task_struct *p)
+{
+ struct shared_runq *shared_runq;
+
+ shared_runq = rq_shared_runq(rq);
+ raw_spin_lock(&shared_runq->lock);
+ list_add_tail(&p->shared_runq_node, &shared_runq->list);
+ raw_spin_unlock(&shared_runq->lock);
+}

static void shared_runq_enqueue_task(struct rq *rq, struct task_struct *p)
-{}
+{
+ /*
+ * Only enqueue the task in the shared runqueue if:
+ *
+ * - SHARED_RUNQ is enabled
+ * - The task isn't pinned to a specific CPU
+ */
+ if (p->nr_cpus_allowed == 1)
+ return;
+
+ shared_runq_push_task(rq, p);
+}

static int shared_runq_pick_next_task(struct rq *rq, struct rq_flags *rf)
{
- return 0;
+ struct task_struct *p = NULL;
+ struct rq *src_rq;
+ struct rq_flags src_rf;
+ int ret = -1;
+
+ p = shared_runq_pop_task(rq);
+ if (!p)
+ return 0;
+
+ rq_unpin_lock(rq, rf);
+ raw_spin_rq_unlock(rq);
+
+ src_rq = task_rq_lock(p, &src_rf);
+
+ if (task_on_rq_queued(p) && !task_on_cpu(src_rq, p)) {
+ update_rq_clock(src_rq);
+ src_rq = move_queued_task(src_rq, &src_rf, p, cpu_of(rq));
+ ret = 1;
+ }
+
+ if (src_rq != rq) {
+ task_rq_unlock(src_rq, p, &src_rf);
+ raw_spin_rq_lock(rq);
+ } else {
+ rq_unpin_lock(rq, &src_rf);
+ raw_spin_unlock_irqrestore(&p->pi_lock, src_rf.flags);
+ }
+ rq_repin_lock(rq, rf);
+
+ return ret;
}

static void shared_runq_dequeue_task(struct task_struct *p)
-{}
+{
+ struct shared_runq *shared_runq;
+
+ if (!list_empty(&p->shared_runq_node)) {
+ shared_runq = rq_shared_runq(task_rq(p));
+ raw_spin_lock(&shared_runq->lock);
+ /*
+ * Need to double-check for the list being empty to avoid
+ * racing with the list being drained on the domain recreation
+ * or SHARED_RUNQ feature enable / disable path.
+ */
+ if (likely(!list_empty(&p->shared_runq_node)))
+ list_del_init(&p->shared_runq_node);
+ raw_spin_unlock(&shared_runq->lock);
+ }
+}

/*
* For asym packing, by default the lower numbered CPU has higher priority.
@@ -12093,6 +12308,16 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
rcu_read_lock();
sd = rcu_dereference_check_sched_domain(this_rq->sd);

+ /*
+ * Skip <= LLC domains as they likely won't have any tasks if the
+ * shared runq is empty.
+ */
+ if (sched_feat(SHARED_RUNQ)) {
+ sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
+ if (likely(sd))
+ sd = sd->parent;
+ }
+
if (!READ_ONCE(this_rq->rd->overload) ||
(sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {

@@ -12969,6 +13194,7 @@ DEFINE_SCHED_CLASS(fair) = {

.task_dead = task_dead_fair,
.set_cpus_allowed = set_cpus_allowed_common,
+ .update_domains = update_domains_fair,
#endif

.task_tick = task_tick_fair,
@@ -13035,6 +13261,7 @@ __init void init_sched_fair_class(void)
{
#ifdef CONFIG_SMP
int i;
+ struct shared_runq *shared_runq;

for_each_possible_cpu(i) {
zalloc_cpumask_var_node(&per_cpu(load_balance_mask, i), GFP_KERNEL, cpu_to_node(i));
@@ -13044,6 +13271,9 @@ __init void init_sched_fair_class(void)
INIT_CSD(&cpu_rq(i)->cfsb_csd, __cfsb_csd_unthrottle, cpu_rq(i));
INIT_LIST_HEAD(&cpu_rq(i)->cfsb_csd_list);
#endif
+ shared_runq = &per_cpu(shared_runqs, i);
+ INIT_LIST_HEAD(&shared_runq->list);
+ raw_spin_lock_init(&shared_runq->lock);
}

open_softirq(SCHED_SOFTIRQ, run_rebalance_domains);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a484bb527ee4..3665dd935649 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -487,9 +487,11 @@ extern int sched_group_set_idle(struct task_group *tg, long idle);
#ifdef CONFIG_SMP
extern void set_task_rq_fair(struct sched_entity *se,
struct cfs_rq *prev, struct cfs_rq *next);
+extern void sched_update_domains(void);
#else /* !CONFIG_SMP */
static inline void set_task_rq_fair(struct sched_entity *se,
struct cfs_rq *prev, struct cfs_rq *next) { }
+static inline void sched_update_domains(void) {}
#endif /* CONFIG_SMP */
#endif /* CONFIG_FAIR_GROUP_SCHED */

@@ -578,6 +580,7 @@ struct cfs_rq {
#endif

#ifdef CONFIG_SMP
+ struct shared_runq *shared_runq;
/*
* CFS load tracking
*/
@@ -2282,6 +2285,7 @@ struct sched_class {
void (*rq_offline)(struct rq *rq);

struct rq *(*find_lock_rq)(struct task_struct *p, struct rq *rq);
+ void (*update_domains)(void);
#endif

void (*task_tick)(struct rq *rq, struct task_struct *p, int queued);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 05a5bc678c08..8aaf644d4f2c 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2576,6 +2576,8 @@ int __init sched_init_domains(const struct cpumask *cpu_map)
doms_cur = &fallback_doms;
cpumask_and(doms_cur[0], cpu_map, housekeeping_cpumask(HK_TYPE_DOMAIN));
err = build_sched_domains(doms_cur[0], NULL);
+ if (!err)
+ sched_update_domains();

return err;
}
@@ -2741,7 +2743,7 @@ void partition_sched_domains_locked(int ndoms_new, cpumask_var_t doms_new[],
dattr_cur = dattr_new;
ndoms_cur = ndoms_new;

- update_sched_domain_debugfs();
+ sched_update_domains();
}

/*
--
2.41.0