linux-kernel - Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227] 6.4.7-rc1 review)

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Date:   Tue, 1 Aug 2023 21:08:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Roy Hopkins <rhopkins@...e.de>,
        Joel Fernandes <joel@...lfernandes.org>, paulmck@...nel.org,
        Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, patches@...ts.linux.dev,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, shuah@...nel.org, patches@...nelci.org,
        lkft-triage@...ts.linaro.org, jonathanh@...dia.com,
        f.fainelli@...il.com, sudipm.mukherjee@...il.com,
        srw@...dewatkins.net, rwarsow@....de, conor@...nel.org,
        rcu@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)

On Tue, Aug 01, 2023 at 10:32:45AM -0700, Guenter Roeck wrote:
> On 7/31/23 14:15, Peter Zijlstra wrote:
> > On Mon, Jul 31, 2023 at 09:34:29AM -0700, Guenter Roeck wrote:
> > > > Ha!, I was poking around the same thing. My hack below seems to (so far,
> > > > <20 boots) help things.
> > > > 
> > > 
> > > So, dumb question:
> > > How comes this bisects to "sched/fair: Remove sched_feat(START_DEBIT)" ?
> > 
> > That commit changes the timings of things; dumb luck otherwise.
> 
> Kind of scary. So I only experienced the problem because the START_DEBIT patch
> happened to be queued roughly at the same time, and it might otherwise have
> found its way unnoticed into the upstream kernel. That makes me wonder if this
> or other similar patches may uncover similar problems elsewhere in the kernel
> (i.e., either hide new or existing race conditions or expose existing ones).
> 
> This in turn makes me wonder if it would be possible to define a test which
> would uncover such problems without the START_DEBIT patch. Any idea ?

IIRC some of the thread sanitizers use breakpoints to inject random
sleeps, specifically to tickle races.