--- /dev/null
+From 76f970ce51c80f625eb6ddbb24e9cb51b977b598 Mon Sep 17 00:00:00 2001
+From: Dietmar Eggemann <dietmar.eggemann@arm.com>
+Date: Fri, 14 Mar 2025 16:13:45 +0100
+Subject: Revert "sched/core: Reduce cost of sched_move_task when config autogroup"
+
+From: Dietmar Eggemann <dietmar.eggemann@arm.com>
+
+commit 76f970ce51c80f625eb6ddbb24e9cb51b977b598 upstream.
+
+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@amazon.com
+
+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@amazon.com>
+Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
+Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
+Signed-off-by: Ingo Molnar <mingo@kernel.org>
+Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
+Tested-by: Hagar Hemdan <hagarhem@amazon.com>
+Cc: Linus Torvalds <torvalds@linux-foundation.org>
+Link: https://lore.kernel.org/r/20250314151345.275739-1-dietmar.eggemann@arm.com
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ kernel/sched/core.c | 21 +++------------------
+ 1 file changed, 3 insertions(+), 18 deletions(-)
+
+--- a/kernel/sched/core.c
++++ b/kernel/sched/core.c
+@@ -9010,7 +9010,7 @@ void sched_release_group(struct task_gro
+ 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;
+
+@@ -9022,13 +9022,7 @@ static struct task_group *sched_get_task
+ 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)
+@@ -9049,20 +9043,11 @@ void sched_move_task(struct task_struct
+ {
+ 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);
+@@ -9073,7 +9058,7 @@ void sched_move_task(struct task_struct
+ if (running)
+ put_prev_task(rq, tsk);
+
+- sched_change_group(tsk, group);
++ sched_change_group(tsk);
+ if (!for_autogroup)
+ scx_cgroup_move_task(tsk);
+
projects / thirdparty / kernel / stable-queue.git / commitdiff
