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

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Message-ID: <a05743a3-4dec-6af7-302f-d1d2a0db7d3e@roeck-us.net>
Date:   Tue, 1 Aug 2023 10:32:45 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Peter Zijlstra <peterz@...radead.org>
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>
Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)

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 ?

Guenter