Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 1 Aug 2023 21:08:52 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Roy Hopkins <rhopkins@suse.de>,
        Joel Fernandes <joel@joelfernandes.org>, paulmck@kernel.org,
        Pavel Machek <pavel@denx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, patches@lists.linux.dev,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, shuah@kernel.org, patches@kernelci.org,
        lkft-triage@lists.linaro.org, jonathanh@nvidia.com,
        f.fainelli@gmail.com, sudipm.mukherjee@gmail.com,
        srw@sladewatkins.net, rwarsow@gmx.de, conor@kernel.org,
        rcu@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>
Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)
Message-ID: <20230801190852.GG11704@hirez.programming.kicks-ass.net>
References: <3da81a5c-700b-8e21-1bde-27dd3a0b8945@roeck-us.net>
 <20230731141934.GK29590@hirez.programming.kicks-ass.net>
 <20230731143954.GB37820@hirez.programming.kicks-ass.net>
 <f5a18aa3-9db7-6ad2-33d5-3335a18e4e2f@roeck-us.net>
 <20230731145232.GM29590@hirez.programming.kicks-ass.net>
 <7ff2a2393d78275b14ff867f3af902b5d4b93ea2.camel@suse.de>
 <20230731161452.GA40850@hirez.programming.kicks-ass.net>
 <baa58a8e-54f0-2309-b34e-d62999a452a1@roeck-us.net>
 <20230731211517.GA51835@hirez.programming.kicks-ass.net>
 <a05743a3-4dec-6af7-302f-d1d2a0db7d3e@roeck-us.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <a05743a3-4dec-6af7-302f-d1d2a0db7d3e@roeck-us.net>
Precedence: bulk

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.