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Message-ID: <174198389307.14745.11414256611900922252.tip-bot2@tip-bot2>
Date: Fri, 14 Mar 2025 20:24:53 -0000
From: "tip-bot2 for Dietmar Eggemann" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Hagar Hemdan <hagarhem@...zon.com>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: sched/core] Revert "sched/core: Reduce cost of sched_move_task
when config autogroup"
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 3b15262e0019eb4bb212215951a45559d29381fc
Gitweb: https://git.kernel.org/tip/3b15262e0019eb4bb212215951a45559d29381fc
Author: Dietmar Eggemann <dietmar.eggemann@....com>
AuthorDate: Fri, 14 Mar 2025 16:13:45 +01:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 14 Mar 2025 21:13:19 +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) [1].
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.
[1] https://lkml.kernel.org/r/20250205151026.13061-1-hagarhem@amazon.com
Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Tested-by: Hagar Hemdan <hagarhem@...zon.com>
Link: https://lore.kernel.org/r/20250314151345.275739-1-dietmar.eggemann@arm.com
---
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 03d7b63..17e16a1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9015,7 +9015,7 @@ 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;
@@ -9027,13 +9027,7 @@ 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)
@@ -9054,20 +9048,11 @@ 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);
@@ -9078,7 +9063,7 @@ 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);
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