Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a configurable timer

[Huang Rui <ray.huang@amd.com>]

On Tue, Jun 09, 2015 at 05:29:52PM +0800, Peter Zijlstra wrote:
> On Tue, Jun 09, 2015 at 11:13:40AM +0800, Huang Rui wrote:
> > +static void delay_mwaitx(unsigned long __loops)
> > +{
> > +	u32 end, now, delay, addr;
> > +
> > +	delay = __loops;
> > +	rdtsc_barrier();
> > +	rdtscl(end);
> > +	end += delay;
> > +
> > +	while (1) {
> > +		__monitorx(&addr, 0, 0);
> > +		mwaitx(delay, true);
> > +
> > +		rdtsc_barrier();
> > +		rdtscl(now);
> > +		if (end <= now)
> > +			break;
> > +		delay = end - now;
> > +	}
> 
> How about you think instead and do something like:
> 
> 	rdtsc(start);
> 	rdtsc_barrier();
> 
> 	for (;;) {
> 		delay = min(MWAIT_MAX_LOOPS, loops);
> 
> 		__monitorx(&addr, 0, 0);
> 		mwaitx(delay, true);
> 
> 		rdtsc_barrier();
> 		rdtsc(end);
> 		rdtsc_barrier();
> 
> 		loops -= end - start;
> 		if (loops <= 0)
> 			break;
> 
> 		start = end;
> 	}
> 

It looks better, thanks. :) 

> > +}
> > +
> > +/*
> >   * Since we calibrate only once at boot, this
> >   * function should be set once at boot and not changed
> >   */
> > @@ -118,7 +145,12 @@ int read_current_timer(unsigned long *timer_val)
> >  
> >  void __delay(unsigned long loops)
> >  {
> > -	delay_fn(loops);
> > +	if (loops > MWAITX_MAX_LOOPS ||
> > +			!static_cpu_has_safe(X86_FEATURE_MWAITT) ||
> > +			boot_option_delay != DELAY_MWAITX)
> > +		delay_fn(loops);
> > +	else
> > +		delay_mwaitx(loops);
> >  }
> 
> Then you can do away with that fallback entirely.

OK, I will submit V3 to remove kernel parameter and with the change of
delay method.

Thanks,
Rui