linux-kernel - Re: [patch] CFS scheduler, -v19

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Date:	Thu, 19 Jul 2007 10:16:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ian Kent <raven@...maw.net>, Chuck Ebbert <cebbert@...hat.com>,
	Bill Davidsen <davidsen@....com>, linux-kernel@...r.kernel.org
Subject: Re: [patch] CFS scheduler, -v19


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > ah! It passes in a low-res time source into a high-res time 
> > interface (pthread_cond_timedwait()). Could you change the 
> > time(NULL) + 1 to time(NULL) + 2, or change it to:
> > 
> > 	gettimeofday(&wait, NULL);
> > 	wait.tv_sec++;
> 
> This is wrong. It's wrong for two reasons:
> 
>  - it really shouldn't be needed. I don't think "time()" has to be 
>    *exactly* in sync, but I don't think it can be off by a third of a 
>    second or whatever (as the "30% CPU load" would seem to imply)
> 
>  - gettimeofday works on a timeval, pthread_cond_timedwait() works on a 
>    timespec.

ah, i didnt notice that automount mixed up timespec with timeval! That 
is nasty and the tv_nsec field (which really is ts_usec to 
pthread_cond_timewait()) must stay cleared - or rather, to avoid bugs of 
this type, a timespec variable should be used for all this.

> So if it actually makes a difference, it makes a difference for the 
> *wrong* reason: the time is still totally nonsensical in the tv_nsec 
> field (because it actually got filled in with msecs!), but now the 
> tv_sec field is in sync, so it hides the bug.
> 
> Anyway, hopefully the patch below might help. But we probably should make 
> this whole thing a much more generic routine (ie we have our internal 
> "getnstimeofday()" that still is missing the second-overflow logic, and 
> that is quite possibly the one that triggers the "30% off" behaviour).

yeah, i'll generalize it, but our internal getnstimeofday() used on most 
architectures is using __get_realtime_clock_ns(), and the patch you 
attached already adds the second-overflow logic to it.

there are two versions of getnstimeofday(), a TIME_INTERPOLATION one and 
a !TIME_INTERPOLATION one. TIME_INTERPOLATION is only used on ia64 at 
the moment - and that one indeed does not have the second overflow 
logic.

> Ingo, I'd suggest:
>  - ger rid of "timespec_add_ns()", or at least make it return a return 
>    value for when it overflows.
>  - make all the people who overflow into tv_sec call a "fix_up_seconds()" 
>    thing that does the xtime overflow handling.

ok, i'll do something clean.

	Ingo
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