linux-kernel - Re: [PATCH] Kick CPUS that might be sleeping in cpus_idle

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Date:	Wed, 9 Jan 2008 19:05:17 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	mingo@...e.hu, tglx@...utronix.de, len.brown@...el.com,
	venkatesh.pallipadi@...el.com, abelay@...ell.com,
	a.p.zijlstra@...llo.nl, ak@...e.de
Subject: Re: [PATCH] Kick CPUS that might be sleeping in cpus_idle_wait


On Wed, 9 Jan 2008, Andrew Morton wrote:

>
> > Subject: [PATCH] Kick CPUS that might be sleeping in cpus_idle_wait
>
> s/cpus_/cpu_/

I've been up to 4am writing patches. I must be seeing double :-/

> >  {
> >  	unsigned int cpu, this_cpu = get_cpu();
> > @@ -228,6 +232,13 @@ void cpu_idle_wait(void)
> >  				cpu_clear(cpu, map);
> >  		}
> >  		cpus_and(map, map, cpu_online_map);
> > +		/*
> > +		 * We waited 1 sec, if a CPU still did not call idle
> > +		 * it may be because it is in idle and not waking up
> > +		 * because it has nothing to do.
> > +		 * Give all the remaining CPUS a kick.
> > +		 */
> > +		smp_call_function_mask(map, do_nothing, 0, 0);
> >  	} while (!cpus_empty(map));
> >
> >  	set_cpus_allowed(current, tmp);
>
> This seems rather hacky.  Although it may turn out to be the most efficient
> fix, dunno.

s/seems/is/

>
> I'd have thought that the right fix would be to plug the race which you
> described at the top-of-thread.  That might require some redesign, but it
> sounds like the design is wrong anyway.
>
> Maybe your proposed fix is suitable for a 2.6.24 bandaid..

I was thinking the same thing.

>
> <looks at cpu_idle_wait()>
>
> <pokes his tongue out at the person who put in a global,
> exported-to-modules interface and didn't bother documenting it>
>
> OK, it's called infrequently, so a few extra IPIs there won't hurt.
>
>
> btw, it's pretty damn sad that cpu_idle_wait() will always stall for at
> least one second.  That's a huge amount of time and I bet it's thousands of
> times longer than is actually needed..
>

I didn't like that either. But I was focusing on something else, and I was
getting sick and tired of my box hanging on bootup every once in a while
(usually when I reboot and walk away, just to come back to find the box
hung).

So this was my band-aid, and since it was only happening on the box
running with my patches, I thought it may have been something I did. But
then it finally hung on a reboot to a vanilla kernel, so I decided to at
least send my band-aid out.

If anything, this should get some notice and we can have a proper fix for
.25.

-- Steve

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