linux-kernel - Re: [PATCH] x86: enable preemption in delay

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Message-ID: <Pine.LNX.4.58.0805250905080.20291@gandalf.stny.rr.com>
Date:	Sun, 25 May 2008 09:07:15 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>, akpm@...l.org,
	Ingo Molnar <mingo@...e.hu>,
	Clark Williams <clark.williams@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Gregory Haskins <ghaskins@...ell.com>, Andi Kleen <ak@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: enable preemption in delay


On Sun, 25 May 2008, Thomas Gleixner wrote:
> > -	preempt_disable();		/* TSC's are per-cpu */
> > +	preempt_disable();
> > +	cpu = smp_processor_id();
> >  	rdtscl(bclock);
> >  	do {
> >  		rep_nop();
> >  		rdtscl(now);
> > +		/* Allow RT tasks to run */
> > +		preempt_enable();
> > +		preempt_disable();
> > +		/*
> > +		 * It is possible that we moved to another CPU,
> > +		 * and since TSC's are per-cpu we need to
> > +		 * calculate that. The delay must guarantee that
> > +		 * we wait "at least" the amount of time. Being
> > +		 * moved to another CPU could make the wait longer
> > +		 * but we just need to make sure we waited long
> > +		 * enough. Rebalance the counter for this CPU.
> > +		 */
> > +		if (unlikely(cpu != smp_processor_id())) {
>
> Eeek, once you migrated you do this all the time. you need to update
> cpu here.

Good catch! I'll update that.

>
> > +			if ((now-bclock) >= loops)
> > +				break;
>
> Also this is really dangerous with unsynchronized TSCs. You might get
> migrated and return immediately because the TSC on the other CPU is
> far ahead.

No it isn't ;-)

The now and bclock are both from before the migration. The cpus were the
same becaues we were under preempt disbled at the time. I recalculate
after the change has been noticed.

But you are right, I forgot to update cpu. :-/

>
> What you really want is something like the patch below, but we should
> reuse the sched_clock_cpu() thingy to make that simpler. Looking into
> that right now.
>

Sure, but this should be simple enough.

-- Steve

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