linux-kernel - Re: [powerpc][next-20210621] WARNING at kernel/sched/fair.c:3277 during boot

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Date:   Wed, 23 Jun 2021 19:37:23 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Odin Ugedal <odin@...d.al>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [powerpc][next-20210621] WARNING at kernel/sched/fair.c:3277
 during boot

ons. 23. jun. 2021 kl. 19:27 skrev Vincent Guittot <vincent.guittot@...aro.org>:
>
> On Wed, 23 Jun 2021 at 18:55, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Wed, 23 Jun 2021 at 18:46, Sachin Sant <sachinp@...ux.vnet.ibm.com> wrote:
> > >
> > >
> > > > Ok. This becomes even more weird. Could you share your config file and more details about
> > > > you setup ?
> > > >
> > > > Have you applied the patch below ?
> > > > https://lore.kernel.org/lkml/20210621174330.11258-1-vincent.guittot@linaro.org/
> > > >
> > > > Regarding the load_avg warning, I can see possible problem during attach. Could you add
> > > > the patch below. The load_avg warning seems to happen during boot and sched_entity
> > > > creation.
> > > >
> > >
> > > Here is a summary of my testing.
> > >
> > > I have a POWER box with PowerVM hypervisor. On this box I have a logical partition(LPAR) or guest
> > > (allocated with 32 cpus 90G memory) running linux-next.
> > >
> > > I started with a clean slate.
> > > Moved to linux-next 5.13.0-rc7-next-20210622 as base code.
> > > Applied patch #1 from Vincent which contains changes to dequeue_load_avg()
> > > Applied patch #2 from Vincent which contains changes to enqueue_load_avg()
> > > Applied patch #3 from Vincent which contains changes to attach_entity_load_avg()
> > > Applied patch #4 from https://lore.kernel.org/lkml/20210621174330.11258-1-vincent.guittot@linaro.org/
> > >
> > > With these changes applied I was still able to recreate the issue. I could see kernel warning
> > > during boot.
> > >
> > > I then applied patch #5 from Odin which contains changes to update_cfs_rq_load_avg()
> > >
> > > With all the 5 patches applied I was able to boot the kernel without any warning messages.
> > > I also ran scheduler related tests from ltp (./runltp -f sched) . All tests including cfs_bandwidth01
> > > ran successfully. No kernel warnings were observed.
> >
> > ok so Odin's patch fixes the problem which highlights that we
> > overestimate _sum or don't sync _avg and _sum correctly
> >
> > I'm going to look at this further
>
> The problem is  "_avg * divider" makes the assumption that all pending
> contrib are not null contributions whereas they can be null.

Yeah.

> Odin patch is the right way to fix this. Other patches should not be
> useful for your problem

Ack. As I see it, given how PELT works now, it is the only way to
mitigate it (without doing a lot of extra PELT stuff).
Will post it as a patch together with a proper message later today or tomorrow.

>
> >
> > >
> > > Have also attached .config in case it is useful. config has CONFIG_HZ_100=y
> >
> > Thanks, i will have a look
> >
> > >
> > > Thanks
> > > -Sachin
> > >

Thanks for reporting Sachin!

Thanks
Odin