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

[Peter Zijlstra]

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.