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

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Message-ID: <20080525180122.GA4247@ucw.cz>
Date:	Sun, 25 May 2008 20:01:23 +0200
From:	Pavel Machek <pavel@....cz>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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

Hi!

> > +		if (unlikely(cpu != smp_processor_id())) {
> 
> Eeek, once you migrated you do this all the time. you need to update
> cpu here.
> 
> > +			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.
> 
> 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.

> @@ -40,17 +40,51 @@ static void delay_loop(unsigned long loops)
>  		:"0" (loops));
>  }
>  
> +/*
> + * 5 usec on a 1GHZ machine. Not necessarily correct, but not too long
> + * either.
> + */
> +#define TSC_MIGRATE_COUNT 5000
> +
>  /* TSC based delay: */
>  static void delay_tsc(unsigned long loops)
>  {
>  	unsigned long bclock, now;
> +	int cpu;
>  
> -	preempt_disable();		/* TSC's are per-cpu */
> +	preempt_disable();
> +	cpu = smp_processor_id();
>  	rdtscl(bclock);
>  	do {
>  		rep_nop();
> -		rdtscl(now);
> -	} while ((now-bclock) < loops);
> +
> +		/* 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())) {
> +			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?
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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