Messages in this thread Patch in this message |  | | Date | Mon, 10 Feb 2025 11:38:51 +0100 | | Subject | Re: BUG Report: Fork benchmark drop by 30% on aarch64 | | From | Dietmar Eggemann <> |
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On 07/02/2025 12:07, Hagar Hemdan wrote: > On Fri, Feb 07, 2025 at 10:14:54AM +0100, Dietmar Eggemann wrote: >> Hi Hagar, >> >> On 05/02/2025 16:10, Hagar Hemdan wrote: >>> Hi, >>> >>> There is about a 30% drop in fork benchmark [1] on aarch64 and a 10% >>> drop on x86_64 using kernel v6.13.1. >>> >>> Git bisect pointed to commit eff6c8ce8d4d ("sched/core: Reduce cost >>> of sched_move_task when config autogroup") which merged starting >>> v6.4-rc1. >>> >>> The regression only happens when number of CPUs is equal to number >>> of threads [2] that fork test is creating which means it's only visible >>> under CPU contention. >>> >>> I used m6g.xlarge AWS EC2 Instance with 4 vCPUs and 16 GiB RAM for ARM64 >>> and m6a.xlarge with also 4 vCPUs and 16 GiB RAM for x86_64. >>> >>> I noticed this regression exists only when autogroup config is enabled. >> >> So '# CONFIG_SCHED_AUTOGROUP is not set' in .config so we have: >> >> static inline void sched_autogroup_exit_task(struct task_struct *p) { } >> >> I.e. doing a 'echo 0 > /proc/sys/kernel/sched_autogroup_enabled' still >> shows this issue? > yes, when I do 'echo 0 | sudo tee /proc/sys/kernel/sched_autogroup_enabled', > It behaves like the disable 'CONFIG_SCHED_AUTOGROUP'.
OK.
>>> Run the fork test with these combinations and autogroup is enabled: >>> >>> Arch | commit eff6c8ce8d4d | Fork Result (lps) | %Cpu(s) >>> ----------+---------------------+--------------------+------------------ >>> aarch64 | without | 28677.0 | 3.2 us, 96.7 sy >>> aarch64 | with | 19860.7 (30% drop) | 2.7 us, 79.4 sy >>> x86_64 | without | 27776.2 | 3.1 us, 96.9 sy >>> x86_64 | with | 25020.6 (10% drop) | 4.1 us, 93.2 sy >>> ----------+---------------------+--------------------+------------------ >> >> Can you rerun with: >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 3e5a6bf587f9..62cc50c79a78 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -9057,7 +9057,7 @@ void sched_move_task(struct task_struct *tsk) >> * group changes. >> */ >> group = sched_get_task_group(tsk); >> - if (group == tsk->sched_task_group) >> + if ((group == tsk->sched_task_group) && !(tsk->flags & PF_EXITING)) >> return; > I tried that and I see it fixed the regression and the cpu utilization > is 100% with it.
W/ this one, we force the 'spawn' tasks into:
sched_change_group() -> task_change_group_fair()
> I'd like to ask if this like reverting the patch in case of exit path > and also means the enqueue/dequeue are needed in case of task exiting, > right?
The 'spawn' tasks in sched_move_task() are 'running' and 'queued' so we call dequeue_task(), put_prev_task(), enqueue_task() and set_next_task().
I guess what we need here is the cfs_rq->avg.load_avg (cpu_load() in case of root tg) update in:
task_change_group_fair() -> detach_task_cfs_rq() -> ..., attach_task_cfs_rq() -> ...
since this is used for WF_FORK, WF_EXEC handling in wakeup:
select_task_rq_fair() -> sched_balance_find_dst_cpu() -> sched_balance_find_dst_group_cpu()
in form of 'least_loaded_cpu' and 'load = cpu_load(cpu_rq(i)'.
You mentioned AutoGroups (AG). I don't see this issue on my Debian 12 Juno-r0 Arm64 board. When I run w/ AG, 'group' is '/' and 'tsk->sched_task_group' is '/autogroup-x' so the condition 'if (group == tsk->sched_task_group)' isn't true in sched_move_task(). If I disable AG then they match "/" == "/".
I assume you run Ubuntu on your AWS instances? What kind of 'cgroup/taskgroup' related setup are you using?
Can you run w/ this debug snippet w/ and w/o AG enabled?
-->8--
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3e5a6bf587f9..c696740177b7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -9035,6 +9035,8 @@ static void sched_change_group(struct task_struct *tsk, struct task_group *group set_task_rq(tsk, task_cpu(tsk)); } +extern void task_group_path(struct task_group *tg, char *path, int plen); + /* * Change task's runqueue when it moves between groups. * @@ -9048,6 +9050,8 @@ void sched_move_task(struct task_struct *tsk) DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK; struct task_group *group; struct rq *rq; + int cpu = cpu_of(rq_of(cfs_rq_of(&tsk->se))); + char buf_1[64], buf_2[64]; CLASS(task_rq_lock, rq_guard)(tsk); rq = rq_guard.rq; @@ -9057,6 +9061,13 @@ void sched_move_task(struct task_struct *tsk) * group changes. */ group = sched_get_task_group(tsk); + + task_group_path(group, buf_1, sizeof(buf_1)); + task_group_path(tsk->sched_task_group, buf_2, sizeof(buf_2)); + + trace_printk("%s %d cpu=%d group=%s tsk->sched_task_group=%s\n", + tsk->comm, tsk->pid, cpu, buf_1, buf_2); + if (group == tsk->sched_task_group) return; diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index a1be00a988bf..c967d55f971f 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -701,7 +701,9 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group static DEFINE_SPINLOCK(sched_debug_lock); static char group_path[PATH_MAX]; -static void task_group_path(struct task_group *tg, char *path, int plen) +extern void task_group_path(struct task_group *tg, char *path, int plen); + +void task_group_path(struct task_group *tg, char *path, int plen) { if (autogroup_path(tg, path, plen)) return; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 26958431deb7..f90d28be9695 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -13198,9 +13198,16 @@ void init_cfs_rq(struct cfs_rq *cfs_rq) #endif } +extern void task_group_path(struct task_group *tg, char *path, int plen); + #ifdef CONFIG_FAIR_GROUP_SCHED static void task_change_group_fair(struct task_struct *p) { + int cpu = cpu_of(rq_of(cfs_rq_of(&p->se))); + struct task_group *tg = task_group(p); + unsigned long cfs_rq_load_avg_pre, cfs_rq_load_avg_post; + char buf[64]; + /* * We couldn't detach or attach a forked task which * hasn't been woken up by wake_up_new_task(). @@ -13208,14 +13215,33 @@ static void task_change_group_fair(struct task_struct *p) if (READ_ONCE(p->__state) == TASK_NEW) return; + task_group_path(tg, buf, sizeof(buf)); + cfs_rq_load_avg_pre = cfs_rq_of(&p->se)->avg.load_avg; + detach_task_cfs_rq(p); + cfs_rq_load_avg_post = cfs_rq_of(&p->se)->avg.load_avg; + + trace_printk("%s %d (d) cpu=%d tg=%s load_avg=%lu->%lu\n", + p->comm, p->pid, cpu, buf, cfs_rq_load_avg_pre, + cfs_rq_load_avg_post); + #ifdef CONFIG_SMP /* Tell se's cfs_rq has been changed -- migrated */ p->se.avg.last_update_time = 0; #endif set_task_rq(p, task_cpu(p)); + + task_group_path(tg, buf, sizeof(buf)); + cfs_rq_load_avg_pre = cfs_rq_of(&p->se)->avg.load_avg; + attach_task_cfs_rq(p); + + cfs_rq_load_avg_post = cfs_rq_of(&p->se)->avg.load_avg; + + trace_printk("%s %d (a) cpu=%d tg=%s load_avg=%lu->%lu\n", + p->comm, p->pid, cpu, buf, cfs_rq_load_avg_pre, + cfs_rq_load_avg_post); }
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