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

[Andy Lutomirski]

On Jun 9, 2015 2:30 AM, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx> 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();

Other way around.  We really need a function static inline u64
rdtsc_with_barrier().

>
>         for (;;) {
>                 delay = min(MWAIT_MAX_LOOPS, loops);
>
>                 __monitorx(&addr, 0, 0);
>                 mwaitx(delay, true);

I don't like this hack.  The compiler is entirely within is rights to
poke addr's cacheline (i.e. the stack) between the two instructions.
I'd suggest either making the thing a full cacheline long or using a
single asm statement.

Also, "addr" is a bad name for a dummy variable that isn't an address
at all.  How about "dummy"?

>
>                 rdtsc_barrier();
>                 rdtsc(end);
>                 rdtsc_barrier();

The second barrier is unnecessary.

>
>                 loops -= end - start;
>                 if (loops <= 0)
>                         break;
>
>                 start = end;
>         }

--Andy
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