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Date: Wed, 22 Jun 2016 12:28:59 +0200
From: Cyril Hrubis <chrubis@suse.cz>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>, Rik van Riel <riel@redhat.com>,
        Len Brown <lenb@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        George Spelvin <linux@sciencehorizons.net>, Chris Mason <clm@fb.com>,
        Eric Dumazet <edumazet@google.com>, rt@linutronix.de,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, ltp@lists.linux.it,
        Arjan van de Ven <arjan@infradead.org>
Subject: Re: [LTP] [patch V2 00/20] timer: Refactor the timer wheel
Message-ID: <20160622102858.GA13962@rei.lan>
References: <20160617121134.417319325@linutronix.de>
 <1466581044.3188.34.camel@gmail.com>
 <alpine.DEB.2.11.1606221037310.5839@nanos>
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Hi!
> > rtbox:~ # /usr/local/ltp/conformance/interfaces/sigtimedwait/sigtimedwait_1-1.run-test
> > Test FAILED: sigtimedwait() did not return in the required time
> > time_elapsed: 1.197057
> > ...come on, you can do it...
> > rtbox:~ # /usr/local/ltp/conformance/interfaces/sigtimedwait/sigtimedwait_1-1.run-test
> > Test PASSED
> > 
> > #define ERRORMARGIN 0.1
> > ...
> >         if ((time_elapsed > SIGTIMEDWAITSEC + ERRORMARGIN)
> >             || (time_elapsed < SIGTIMEDWAITSEC - ERRORMARGIN)) {
> >                 printf("Test FAILED: sigtimedwait() did not return in "
> >                         "the required time\n");
> >                 printf("time_elapsed: %lf\n", time_elapsed);
> >                 return PTS_FAIL;
> >         }
> > 
> > Looks hohum to me, but gripe did arrive with patch set, so you get a note.
> 
> hohum is a euphemism. That's completely bogus.
> 
> The only guarantee a syscall with timers has is: timer does not fire early.

While this is true, checking with reasonable error margin works just
fine 99% of the time. You cannot really test that timer expires, without
setting arbitrary margin.

Looking into POSIX sigtimedwait() timer should run on CLOCK_MONOTONIC so
we can call clock_getres(CLOCK_MONOTOINC, ...) double or tripple the
value and use it for error margin. And also fix the test to use
the CLOCK_MONOTONIC timer.

And of course the error margin must not be used when we check that the
elapsed time wasn't shorter than we expected.

Does that sound reasonable?

-- 
Cyril Hrubis
chrubis@suse.cz