Re: [patch V2 00/20] timer: Refactor the timer wheel

From: Pavel Machek <pavel@ucw.cz>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Eric Dumazet <edumazet@google.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Chris Mason <clm@fb.com>, Arjan van de Ven <arjan@infradead.org>,
	rt@linutronix.de, Rik van Riel <riel@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	George Spelvin <linux@sciencehorizons.net>,
	Len Brown <lenb@kernel.org>,
	ltp@lists.linux.it
Subject: Re: [patch V2 00/20] timer: Refactor the timer wheel
Date: Sun, 26 Jun 2016 21:00:25 +0200	[thread overview]
Message-ID: <20160626190025.GC11162@amd> (raw)
In-Reply-To: <alpine.DEB.2.11.1606221037310.5839@nanos>

Hi!

> > FWIW, testing with ltp, I noticed a new failure in logs.  It turns out
> > to be intermittent, but the testcase mostly fails.
> 
> You forgot to cc the LTP folks ...
>  
> > 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.

Umm. I'm not sure if you should be designing kernel...

I have alarm clock application. It does sleep(60) many times till its
time to wake me up. I'll be very angry if sleep(60) takes 65 seconds
without some very, very good reason.

So yes, man page says this is the only requirement (and did not you
break it earlier in the patchset, with sleep for 5 days? :-) ), but
no, it is not really the only requirement you have.

You may argue LTP's ERRORMARGIN is too strict. But you can't argue LTP
is completely bogus, and I'd say error margin of 0.1 second is
completely reasonable (*).

If I update my alarm clock application to display seconds, I really
want them with better precision than 0.1 second (*), because 0.1 seconds
is already visible with the naked eye.

Best regards,
								Pavel

(*) on reasonably idle system.
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html