[PATCH v3 7/7] sched: Shard per-LLC shared runqueues

From: David Vernet
Date: Wed Aug 09 2023 - 18:13:19 EST

The SHARED_RUNQ scheduler feature creates a FIFO queue per LLC that
tasks are put into on enqueue, and pulled from when a core in that LLC
would otherwise go idle. For CPUs with large LLCs, this can sometimes
cause significant contention, as illustrated in [0].

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

So as to try and mitigate this contention, we can instead shard the
per-LLC runqueue into multiple per-LLC shards.

While this doesn't outright prevent all contention, it does somewhat mitigate it.
For example, if we run the following schbench command which does almost
nothing other than pound the runqueue:

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

we observe with lockstats that sharding significantly decreases
contention.

3 shards:

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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

No sharding:

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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 host where this was
collected. This could be addressed in future patch sets by adding a
debugfs knob to control the sharding granularity. If we make the shards
even smaller (what's in this patch, i.e. a size of 6), the contention
goes away almost entirely:

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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, but we contend even more on the rq 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
&rq->__lock 47962 [<00000000c38a30f9>] sched_ttwu_pending+0x3d/0x170

In general, the takeaway here is that sharding does help with
contention, but it's not necessarily one size fits all, and it's
workload dependent. For now, let's include sharding to try and avoid
contention, and because it doesn't seem to regress CPUs that don't need
it such as the AMD 7950X.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
---
kernel/sched/fair.c | 149 ++++++++++++++++++++++++++++++-------------
kernel/sched/sched.h | 3 +-
2 files changed, 108 insertions(+), 44 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e740f8da578..d67d86d3bfdf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -143,19 +143,27 @@ __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.
*
+ * struct shared_runq_shard - A structure containing a task list and a spinlock
+ * for a subset of cores in a struct shared_runq.
+ *
* 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.
+ * conserving, by placing waking tasks on a per-LLC FIFO queue shard that can
+ * then be pulled from when another core in the LLC is going to go idle.
+ *
+ * struct rq stores two pointers in its struct cfs_rq:
+ *
+ * 1. The per-LLC struct shared_runq which contains one or more shards of
+ * enqueued tasks.
*
- * 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().
+ * 2. The shard inside of the per-LLC struct shared_runq which contains the
+ * list of runnable tasks for that shard.
+ *
+ * Waking tasks are enqueued in the calling CPU's struct shared_runq_shard in
+ * __enqueue_entity(), and are opportunistically pulled from the shared_runq in
+ * newidle_balance(). Pulling from shards is an O(# shards) operation.
*
* 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
@@ -165,11 +173,12 @@ __setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);
* 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.
+ * A struct shared_runq_shard 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_shard is localized
+ * to a subset of cores on a single LLC (plus other cores in the LLC that pull
+ * from the shard in newidle_balance()), the spinlock will typically only be
+ * contended on workloads that do little else other than hammer the runqueue.
*
* WHY
* ===
@@ -183,11 +192,21 @@ __setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift);
* it, as well as to strike a balance between work conservation, and L3 cache
* locality.
*/
-struct shared_runq {
+struct shared_runq_shard {
struct list_head list;
raw_spinlock_t lock;
} ____cacheline_aligned;

+/* This would likely work better as a configurable knob via debugfs */
+#define SHARED_RUNQ_SHARD_SZ 6
+#define SHARED_RUNQ_MAX_SHARDS \
+ ((NR_CPUS / SHARED_RUNQ_SHARD_SZ) + (NR_CPUS % SHARED_RUNQ_SHARD_SZ != 0))
+
+struct shared_runq {
+ unsigned int num_shards;
+ struct shared_runq_shard shards[SHARED_RUNQ_MAX_SHARDS];
+} ____cacheline_aligned;
+
#ifdef CONFIG_SMP

static DEFINE_PER_CPU(struct shared_runq, shared_runqs);
@@ -197,31 +216,61 @@ static struct shared_runq *rq_shared_runq(struct rq *rq)
return rq->cfs.shared_runq;
}

+static struct shared_runq_shard *rq_shared_runq_shard(struct rq *rq)
+{
+ return rq->cfs.shard;
+}
+
+static int shared_runq_shard_idx(const struct shared_runq *runq, int cpu)
+{
+ return (cpu >> 1) % runq->num_shards;
+}
+
static void shared_runq_reassign_domains(void)
{
int i;
struct shared_runq *shared_runq;
struct rq *rq;
struct rq_flags rf;
+ unsigned int num_shards, shard_idx;
+
+ for_each_possible_cpu(i) {
+ if (per_cpu(sd_llc_id, i) == i) {
+ shared_runq = &per_cpu(shared_runqs, per_cpu(sd_llc_id, i));
+
+ num_shards = per_cpu(sd_llc_size, i) / SHARED_RUNQ_SHARD_SZ;
+ if (per_cpu(sd_llc_size, i) % SHARED_RUNQ_SHARD_SZ)
+ num_shards++;
+ shared_runq->num_shards = num_shards;
+ }
+ }

for_each_possible_cpu(i) {
rq = cpu_rq(i);
shared_runq = &per_cpu(shared_runqs, per_cpu(sd_llc_id, i));

+ shard_idx = shared_runq_shard_idx(shared_runq, i);
rq_lock(rq, &rf);
rq->cfs.shared_runq = shared_runq;
+ rq->cfs.shard = &shared_runq->shards[shard_idx];
rq_unlock(rq, &rf);
}
}

static void __shared_runq_drain(struct shared_runq *shared_runq)
{
- struct task_struct *p, *tmp;
+ unsigned int i;

- 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);
+ for (i = 0; i < shared_runq->num_shards; i++) {
+ struct shared_runq_shard *shard;
+ struct task_struct *p, *tmp;
+
+ shard = &shared_runq->shards[i];
+ raw_spin_lock(&shard->lock);
+ list_for_each_entry_safe(p, tmp, &shard->list, shared_runq_node)
+ list_del_init(&p->shared_runq_node);
+ raw_spin_unlock(&shard->lock);
+ }
}

static void update_domains_fair(void)
@@ -272,35 +321,32 @@ void shared_runq_toggle(bool enabling)
}
}

-static struct task_struct *shared_runq_pop_task(struct rq *rq)
+static struct task_struct *
+shared_runq_pop_task(struct shared_runq_shard *shard, int target)
{
struct task_struct *p;
- struct shared_runq *shared_runq;

- shared_runq = rq_shared_runq(rq);
- if (list_empty(&shared_runq->list))
+ if (list_empty(&shard->list))
return NULL;

- raw_spin_lock(&shared_runq->lock);
- p = list_first_entry_or_null(&shared_runq->list, struct task_struct,
+ raw_spin_lock(&shard->lock);
+ p = list_first_entry_or_null(&shard->list, struct task_struct,
shared_runq_node);
- if (p && is_cpu_allowed(p, cpu_of(rq)))
+ if (p && is_cpu_allowed(p, target))
list_del_init(&p->shared_runq_node);
else
p = NULL;
- raw_spin_unlock(&shared_runq->lock);
+ raw_spin_unlock(&shard->lock);

return p;
}

-static void shared_runq_push_task(struct rq *rq, struct task_struct *p)
+static void shared_runq_push_task(struct shared_runq_shard *shard,
+ 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);
+ raw_spin_lock(&shard->lock);
+ list_add_tail(&p->shared_runq_node, &shard->list);
+ raw_spin_unlock(&shard->lock);
}

static void shared_runq_enqueue_task(struct rq *rq, struct task_struct *p)
@@ -314,7 +360,7 @@ static void shared_runq_enqueue_task(struct rq *rq, struct task_struct *p)
if (p->nr_cpus_allowed == 1)
return;

- shared_runq_push_task(rq, p);
+ shared_runq_push_task(rq_shared_runq_shard(rq), p);
}

static int shared_runq_pick_next_task(struct rq *rq, struct rq_flags *rf)
@@ -322,9 +368,22 @@ static int shared_runq_pick_next_task(struct rq *rq, struct rq_flags *rf)
struct task_struct *p = NULL;
struct rq *src_rq;
struct rq_flags src_rf;
+ struct shared_runq *shared_runq;
+ struct shared_runq_shard *shard;
+ u32 i, starting_idx, curr_idx, num_shards;
int ret = -1;

- p = shared_runq_pop_task(rq);
+ shared_runq = rq_shared_runq(rq);
+ num_shards = shared_runq->num_shards;
+ starting_idx = shared_runq_shard_idx(shared_runq, cpu_of(rq));
+ for (i = 0; i < num_shards; i++) {
+ curr_idx = (starting_idx + i) % num_shards;
+ shard = &shared_runq->shards[curr_idx];
+
+ p = shared_runq_pop_task(shard, cpu_of(rq));
+ if (p)
+ break;
+ }
if (!p)
return 0;

@@ -353,11 +412,11 @@ static int shared_runq_pick_next_task(struct rq *rq, struct rq_flags *rf)

static void shared_runq_dequeue_task(struct task_struct *p)
{
- struct shared_runq *shared_runq;
+ struct shared_runq_shard *shard;

if (!list_empty(&p->shared_runq_node)) {
- shared_runq = rq_shared_runq(task_rq(p));
- raw_spin_lock(&shared_runq->lock);
+ shard = rq_shared_runq_shard(task_rq(p));
+ raw_spin_lock(&shard->lock);
/*
* Need to double-check for the list being empty to avoid
* racing with the list being drained on the domain recreation
@@ -365,7 +424,7 @@ static void shared_runq_dequeue_task(struct task_struct *p)
*/
if (likely(!list_empty(&p->shared_runq_node)))
list_del_init(&p->shared_runq_node);
- raw_spin_unlock(&shared_runq->lock);
+ raw_spin_unlock(&shard->lock);
}
}

@@ -13260,8 +13319,9 @@ void show_numa_stats(struct task_struct *p, struct seq_file *m)
__init void init_sched_fair_class(void)
{
#ifdef CONFIG_SMP
- int i;
+ int i, j;
struct shared_runq *shared_runq;
+ struct shared_runq_shard *shard;

for_each_possible_cpu(i) {
zalloc_cpumask_var_node(&per_cpu(load_balance_mask, i), GFP_KERNEL, cpu_to_node(i));
@@ -13272,8 +13332,11 @@ __init void init_sched_fair_class(void)
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);
+ for (j = 0; j < SHARED_RUNQ_MAX_SHARDS; j++) {
+ shard = &shared_runq->shards[j];
+ INIT_LIST_HEAD(&shard->list);
+ raw_spin_lock_init(&shard->lock);
+ }
}

open_softirq(SCHED_SOFTIRQ, run_rebalance_domains);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3665dd935649..b504f8f4416b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -580,7 +580,8 @@ struct cfs_rq {
#endif

#ifdef CONFIG_SMP
- struct shared_runq *shared_runq;
+ struct shared_runq *shared_runq;
+ struct shared_runq_shard *shard;
/*
* CFS load tracking
*/
--
2.41.0