Re: BUG Report: Fork benchmark drop by 30% on aarch64

[Hagar Hemdan <hagarhem@amazon.com>]

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On Tue, Feb 11, 2025 at 05:27:47PM +0100, Dietmar Eggemann wrote:
> On 10/02/2025 22:31, Hagar Hemdan wrote:
> > On Mon, Feb 10, 2025 at 11:38:51AM +0100, Dietmar Eggemann wrote:
> >> 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:
> 
> [...]
> 
> >> 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?
> > 
> > I'm running AL2023 and use Vanilla kernel 6.13.1 on m6g.xlarge AWS instance.
> > AL2023 uses cgroupv2 by default.
> >>
> >> Can you run w/ this debug snippet w/ and w/o AG enabled?
> > 
> > I have run that and have attached the trace files to this email.
> 
> Thanks!
> 
> So w/ AG you see that 'group' and 'tsk->sched_task_group' are both
> '/user.slice/user-1000.slice/session-1.scope' so we bail for those tasks
> w/o doing the 'cfs_rq->avg.load_avg' update I described above.

yes, both groups are identical so it returns from sched_move_task()
without {de|en}queue and without call task_change_group_fair().
> 
> You said that there is no issue w/o AG. 

To clarify, I meant by there's no regression when autogroup is disabled,
that the fork results w/o AG remain consistent with or without the commit 
"sched/core: Reduce cost of sched_move_task when config autogroup". However,
the fork results are consistently lower when AG disabled compared to when
it's enabled (without commit applied). This is illustrated in the tables
provided in the report.

>Unfortunately your 'w/o AG'
> trace does not contain any evidence that you ran UnixBench's './Run -c 4
> spawn' since there are no lines for tasks with p->comm='spawn'.

You're right, the trace doesn't show tasks with p->comm='spawn' because I ran
the fork benchmark with CONFIG_SCHED_AUTOGROUP disabled and 
sched_autogroup_exit_task() is empty in this case.

> Could you rerun this please. My hunch is that 'group' and
> 'tsk->sched_task_group' differ w/o AG?

I've re-run the fork test with CONFIG_SCHED_AUTOGROUP enabled, but with 
autogroup disabled at runtime using 'echo 0 | sudo tee /proc/sys/kernel
/sched_autogroup_enabled' to avoid empty sched_autogroup_exit_task().
I've attached the new trace file and I observed that both groups still
don't differ.
Have you seen different results when testing this on a Debian 12 
Juno-r0 Arm64 board? 
> [...]

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