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

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Date:	Thu, 23 Jun 2016 10:27:06 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Cyril Hrubis <chrubis@...e.cz>
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

On Wed, 22 Jun 2016, Cyril Hrubis wrote:
> 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.

Err. You know that the timer expired because sigtimedwait() returns
EAGAIN. And the only thing you can reliably check for is that the timer did
not expired to early. Anything else is guesswork and voodoo programming.

Thanks,

	tglx