linux-kernel - Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

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Message-ID: <87le7lkzj6.fsf@oracle.com>
Date: Thu, 15 Feb 2024 16:45:17 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, peterz@...radead.org,
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Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling


Paul E. McKenney <paulmck@...nel.org> writes:

> On Thu, Feb 15, 2024 at 01:24:59PM -0800, Ankur Arora wrote:
>>
>> Paul E. McKenney <paulmck@...nel.org> writes:
>>
>> > On Wed, Feb 14, 2024 at 07:45:18PM -0800, Paul E. McKenney wrote:
>> >> On Wed, Feb 14, 2024 at 06:03:28PM -0800, Ankur Arora wrote:
>> >> >
>> >> > Paul E. McKenney <paulmck@...nel.org> writes:
>> >> >
>> >> > > On Mon, Feb 12, 2024 at 09:55:24PM -0800, Ankur Arora wrote:
>> >> > >> Hi,
>> >> > >>
>> >> > >> This series adds a new scheduling model PREEMPT_AUTO, which like
>> >> > >> PREEMPT_DYNAMIC allows dynamic switching between a none/voluntary/full
>> >> > >> preemption model. However, unlike PREEMPT_DYNAMIC, it doesn't depend
>> >> > >> on explicit preemption points for the voluntary models.
>> >> > >>
>> >> > >> The series is based on Thomas' original proposal which he outlined
>> >> > >> in [1], [2] and in his PoC [3].
>> >> > >>
>> >> > >> An earlier RFC version is at [4].
>> >> > >
>> >> > > This uncovered a couple of latent bugs in RCU due to its having been
>> >> > > a good long time since anyone built a !SMP preemptible kernel with
>> >> > > non-preemptible RCU.  I have a couple of fixes queued on -rcu [1], most
>> >> > > likely for the merge window after next, but let me know if you need
>> >> > > them sooner.
>> >> >
>> >> > Thanks. As you can probably tell, I skipped out on !SMP in my testing.
>> >> > But, the attached diff should tide me over until the fixes are in.
>> >>
>> >> That was indeed my guess.  ;-)
>> >>
>> >> > > I am also seeing OOM conditions during rcutorture testing of callback
>> >> > > flooding, but I am still looking into this.
>> >> >
>> >> > That's on the PREEMPT_AUTO && PREEMPT_VOLUNTARY configuration?
>> >>
>> >> On two of the PREEMPT_AUTO && PREEMPT_NONE configurations, but only on
>> >> two of them thus far.  I am running a longer test to see if this might
>> >> be just luck.  If not, I look to see what rcutorture scenarios TREE10
>> >> and TRACE01 have in common.
>> >
>> > And still TRACE01 and TREE10 are hitting OOMs, still not seeing what
>> > sets them apart.  I also hit a grace-period hang in TREE04, which does
>> > CONFIG_PREEMPT_VOLUNTARY=y along with CONFIG_PREEMPT_AUTO=y.  Something
>> > to dig into more.
>>
>> So, the only PREEMPT_VOLUNTARY=y configuration is TREE04. I wonder
>> if you would continue to hit the TREE04 hang with CONFIG_PREEMTP_NONE=y
>> as well?
>> (Just in the interest of minimizing configurations.)
>
> I would be happy to, but in the spirit of full disclosure...
>
> First, I have seen that failure only once, which is not enough to
> conclude that it has much to do with TREE04.  It might simply be low
> probability, so that TREE04 simply was unlucky enough to hit it first.
> In contrast, I have sufficient data to be reasonably confident that the
> callback-flooding OOMs really do have something to do with the TRACE01 and
> TREE10 scenarios, even though I am not yet seeing what these two scenarios
> have in common that they don't also have in common with other scenarios.
> But what is life without a bit of mystery?  ;-)

:).

> Second, please see the attached tarball, which contains .csv files showing
> Kconfig options and kernel boot parameters for the various torture tests.
> The portions of the filenames preceding the "config.csv" correspond to
> the directories in tools/testing/selftests/rcutorture/configs.

So, at least some of the HZ_FULL=y tests don't run into problems.

> Third, there are additional scenarios hand-crafted by the script at
> tools/testing/selftests/rcutorture/bin/torture.sh.  Thus far, none of
> them have triggered, other than via the newly increased difficulty
> of configurating a tracing-free kernel with which to test, but they
> can still be useful in ruling out particular Kconfig options or kernel
> boot parameters being related to a given issue.
>
> But please do take a look at the .csv files and let me know what
> adjustments would be appropriate given the failure information.

Nothing stands out just yet. Let me start a run here and see if
that gives me some ideas.

I'm guessing the splats don't give any useful information or
you would have attached them ;).

Thanks for testing, btw.

--
ankur