linux-kernel - Re: [PATCH] sched: do not stop ticks when cpu is not idle

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Message-ID: <20080718222754.GE31073@elte.hu>
Date:	Sat, 19 Jul 2008 00:27:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	eric miao <eric.y.miao@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jack Ren <jack.ren@...vell.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Fri, 2008-07-18 at 12:54 +0200, Ingo Molnar wrote:
> > * Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > --- a/kernel/sched.c
> > > +++ b/kernel/sched.c
> > > @@ -4446,7 +4446,8 @@ need_resched_nonpreemptible:
> > >  		rq->nr_switches++;
> > >  		rq->curr = next;
> > >  		++*switch_count;
> > > -
> > > +		if (rq->curr != rq->idle)
> > > +			tick_nohz_restart_sched_tick();
> > >  		context_switch(rq, prev, next); /* unlocks the rq */
> > 
> > hm, one problem i can see is lock dependencies. This code is executed 
> > with the rq lock while tick_nohz_restart_sched_tick() takes hr locks => 
> > not good. So i havent applied this just yet - this needs to be solved 
> > differently.
> 
> Actually, that should work these days...
> 
> Also, I assume Eric actually tested this with lockdep enabled (right, 
> Eric?) and that'll shout - or rather, lockup hard in this case - if 
> you got it wrong.

nope:

[    0.188011] =================================
[    0.188011] [ INFO: inconsistent lock state ]
[    0.188011] 2.6.26-tip-03835-g9d964b9-dirty #20198
[    0.188011] ---------------------------------
[    0.188011] inconsistent {in-hardirq-W} -> {hardirq-on-W} usage.
[    0.188011] swapper/0 [HC0[0]:SC0[0]:HE1:SE1] takes:
[    0.188011]  (&rq->rq_lock_key){+...}, at: [<ffffffff816a4b35>] schedule+0x191/0x900
[    0.188011] {in-hardirq-W} state was registered at:
[    0.188011]   [<ffffffffffffffff>] 0xffffffffffffffff

	Ingo
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