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

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Message-ID: <Pine.LNX.4.58.0805280855250.25179@gandalf.stny.rr.com>
Date:	Wed, 28 May 2008 09:01:06 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pavel Machek <pavel@....cz>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	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, Pavel Machek wrote:
> > +		if (unlikely(cpu != smp_processor_id())) {
> > +			if (loops <= TSC_MIGRATE_COUNT)
> > +				break;
> > +			cpu = smp_processor_id();
> > +			rdtscl(bclock);
> > +			loops -= TSC_MIGRATE_COUNT;
> > +		} else {
> > +			rdtscl(now);
> > +			if ((now - bclock) >= loops)
> > +				break;
> > +			loops -= (now - bclock);
> > +			bclock = now;
>
> What happens with different cpus running on different frequencies...?
> Cpufreq?

It's not even protected with the old code.

inline void __const_udelay(unsigned long xloops)
{
	__delay(((xloops * HZ *
		cpu_data(raw_smp_processor_id()).loops_per_jiffy) >> 32) + 1);
}

Here it calculates the loops_per_jiffy for the CPU and calls into __delay.
But it can easily be preempted here and the delay could run on another
CPU.

-- Steve

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