[tip: sched/urgent] Revert "sched/core: Reduce cost of sched_move_task when config autogroup"
From: tip-bot2 for Dietmar Eggemann
Date: Sat Mar 15 2025 - 05:47:47 EST
The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: 76f970ce51c80f625eb6ddbb24e9cb51b977b598
Gitweb: https://git.kernel.org/tip/76f970ce51c80f625eb6ddbb24e9cb51b977b598
Author: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
AuthorDate: Fri, 14 Mar 2025 16:13:45 +01:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Sat, 15 Mar 2025 10:34:27 +01:00
Revert "sched/core: Reduce cost of sched_move_task when config autogroup"
This reverts commit eff6c8ce8d4d7faef75f66614dd20bb50595d261.
Hazem reported a 30% drop in UnixBench spawn test with commit
eff6c8ce8d4d ("sched/core: Reduce cost of sched_move_task when config
autogroup") on a m6g.xlarge AWS EC2 instance with 4 vCPUs and 16 GiB RAM
(aarch64) (single level MC sched domain):
https://lkml.kernel.org/r/20250205151026.13061-1-hagarhem@xxxxxxxxxx
There is an early bail from sched_move_task() if p->sched_task_group is
equal to p's 'cpu cgroup' (sched_get_task_group()). E.g. both are
pointing to taskgroup '/user.slice/user-1000.slice/session-1.scope'
(Ubuntu '22.04.5 LTS').
So in:
do_exit()
sched_autogroup_exit_task()
sched_move_task()
if sched_get_task_group(p) == p->sched_task_group
return
/* p is enqueued */
dequeue_task() \
sched_change_group() |
task_change_group_fair() |
detach_task_cfs_rq() | (1)
set_task_rq() |
attach_task_cfs_rq() |
enqueue_task() /
(1) isn't called for p anymore.
Turns out that the regression is related to sgs->group_util in
group_is_overloaded() and group_has_capacity(). If (1) isn't called for
all the 'spawn' tasks then sgs->group_util is ~900 and
sgs->group_capacity = 1024 (single CPU sched domain) and this leads to
group_is_overloaded() returning true (2) and group_has_capacity() false
(3) much more often compared to the case when (1) is called.
I.e. there are much more cases of 'group_is_overloaded' and
'group_fully_busy' in WF_FORK wakeup sched_balance_find_dst_cpu() which
then returns much more often a CPU != smp_processor_id() (5).
This isn't good for these extremely short running tasks (FORK + EXIT)
and also involves calling sched_balance_find_dst_group_cpu() unnecessary
(single CPU sched domain).
Instead if (1) is called for 'p->flags & PF_EXITING' then the path
(4),(6) is taken much more often.
select_task_rq_fair(..., wake_flags = WF_FORK)
cpu = smp_processor_id()
new_cpu = sched_balance_find_dst_cpu(..., cpu, ...)
group = sched_balance_find_dst_group(..., cpu)
do {
update_sg_wakeup_stats()
sgs->group_type = group_classify()
if group_is_overloaded() (2)
return group_overloaded
if !group_has_capacity() (3)
return group_fully_busy
return group_has_spare (4)
} while group
if local_sgs.group_type > idlest_sgs.group_type
return idlest (5)
case group_has_spare:
if local_sgs.idle_cpus >= idlest_sgs.idle_cpus
return NULL (6)
Unixbench Tests './Run -c 4 spawn' on:
(a) VM AWS instance (m7gd.16xlarge) with v6.13 ('maxcpus=4 nr_cpus=4')
and Ubuntu 22.04.5 LTS (aarch64).
Shell & test run in '/user.slice/user-1000.slice/session-1.scope'.
w/o patch w/ patch
21005 27120
(b) i7-13700K with tip/sched/core ('nosmt maxcpus=8 nr_cpus=8') and
Ubuntu 22.04.5 LTS (x86_64).
Shell & test run in '/A'.
w/o patch w/ patch
67675 88806
CONFIG_SCHED_AUTOGROUP=y & /sys/proc/kernel/sched_autogroup_enabled equal
0 or 1.
Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@xxxxxxxxxx>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: Hagar Hemdan <hagarhem@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250314151345.275739-1-dietmar.eggemann@xxxxxxx
---
kernel/sched/core.c | 21 +++------------------
1 file changed, 3 insertions(+), 18 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6718990..042351c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9016,7 +9016,7 @@ void sched_release_group(struct task_group *tg)
spin_unlock_irqrestore(&task_group_lock, flags);
}
-static struct task_group *sched_get_task_group(struct task_struct *tsk)
+static void sched_change_group(struct task_struct *tsk)
{
struct task_group *tg;
@@ -9028,13 +9028,7 @@ static struct task_group *sched_get_task_group(struct task_struct *tsk)
tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
struct task_group, css);
tg = autogroup_task_group(tsk, tg);
-
- return tg;
-}
-
-static void sched_change_group(struct task_struct *tsk, struct task_group *group)
-{
- tsk->sched_task_group = group;
+ tsk->sched_task_group = tg;
#ifdef CONFIG_FAIR_GROUP_SCHED
if (tsk->sched_class->task_change_group)
@@ -9055,20 +9049,11 @@ void sched_move_task(struct task_struct *tsk, bool for_autogroup)
{
int queued, running, queue_flags =
DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
- struct task_group *group;
struct rq *rq;
CLASS(task_rq_lock, rq_guard)(tsk);
rq = rq_guard.rq;
- /*
- * Esp. with SCHED_AUTOGROUP enabled it is possible to get superfluous
- * group changes.
- */
- group = sched_get_task_group(tsk);
- if (group == tsk->sched_task_group)
- return;
-
update_rq_clock(rq);
running = task_current_donor(rq, tsk);
@@ -9079,7 +9064,7 @@ void sched_move_task(struct task_struct *tsk, bool for_autogroup)
if (running)
put_prev_task(rq, tsk);
- sched_change_group(tsk, group);
+ sched_change_group(tsk);
if (!for_autogroup)
scx_cgroup_move_task(tsk);