linux-kernel - Re: [LTP] [patch V2 00/20] timer: Refactor the timer wheel

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Date:	Wed, 22 Jun 2016 12:28:59 +0200
From:	Cyril Hrubis <chrubis@...e.cz>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Rik van Riel <riel@...hat.com>, Len Brown <lenb@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	George Spelvin <linux@...encehorizons.net>,
	Chris Mason <clm@...com>, Eric Dumazet <edumazet@...gle.com>,
	rt@...utronix.de, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, ltp@...ts.linux.it,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [LTP] [patch V2 00/20] timer: Refactor the timer wheel

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@...e.cz