Re: [PATCH] selftests/timers/posix_timers: reimplement check_timer_distribution()

From: Mark Brown <broonie@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	John Stultz <jstultz@google.com>, Marco Elver <elver@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	Dmitry Vyukov <dvyukov@google.com>,
	kasan-dev@googlegroups.com, Edward Liaw <edliaw@google.com>,
	Carlos Llamas <cmllamas@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] selftests/timers/posix_timers: reimplement check_timer_distribution()
Date: Thu, 11 Apr 2024 13:41:59 +0100	[thread overview]
Message-ID: <a9a4a964-6f0c-43a5-9fa8-10926d74fbf1@sirena.org.uk> (raw)
In-Reply-To: <20240406150950.GA3060@redhat.com>

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On Sat, Apr 06, 2024 at 05:09:51PM +0200, Oleg Nesterov wrote:
> Thomas says:
> 
> 	The signal distribution test has a tendency to hang for a long
> 	time as the signal delivery is not really evenly distributed. In
> 	fact it might never be distributed across all threads ever in
> 	the way it is written.
> 
> To me even the
> 
> 	This primarily tests that the kernel does not favour any one.
> 
> comment doesn't look right. The kernel does favour a thread which hits
> the timer interrupt when CLOCK_PROCESS_CPUTIME_ID expires.
> 
> The new version simply checks that the group leader sleeping in join()
> never receives SIGALRM, cpu_timer_fire() should always send the signal
> to the thread which burns cpu.
> 
> Without the commit bcb7ee79029d ("posix-timers: Prefer delivery of signals
> to the current thread") the test-case fails immediately, the very 1st tick
> wakes the leader up. Otherwise it quickly succeeds after 100 ticks.

This has landed in -next and is causing warning spam throughout
kselftest when built with clang:

/home/broonie/git/bisect/tools/testing/selftests/kselftest.h:435:6: warning: variable 'major' is used uninitialized whenever '||' condition is true [-Wsometimes-uninitialized]
        if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
            ^~~~~~~~~~~~
/home/broonie/git/bisect/tools/testing/selftests/kselftest.h:438:9: note: uninitialized use occurs here
        return major > min_major || (major == min_major && minor >= min_minor);
               ^~~~~
/home/broonie/git/bisect/tools/testing/selftests/kselftest.h:435:6: note: remove the '||' if its condition is always false
        if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
            ^~~~~~~~~~~~~~~
/home/broonie/git/bisect/tools/testing/selftests/kselftest.h:432:20: note: initialize the variable 'major' to silence this warning
        unsigned int major, minor;
                          ^
                           = 0

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