Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a configurable timer

Borislav Petkov
2015-05-21T17:09:04Z

On Thu, May 21, 2015 at 09:45:10AM -0700, Andy Lutomirski wrote:
> I must be missing something. In this sequence, you're sleeping with
> IF=0 and ECX[0] = 0, so an IRQ won't get handled. Don't we want
> ECX[0] = 1?

Hmm, so actually we don't want to sleep with interrupts disabled. If
ECX[0]=1b, then an interrupt will wake MWAIT. So then you have to do the
loop thing as tglx suggested.

> > The NMI argument is a problem though - if and NMI gets you out of
> > MWAITX, a simple perf tool workload would kill all MWAITX executions.
> > Which is bad. :-\
>
> I'm not sure it's a show-stopper. NMI handlers are meant to be fast.
> If an NMI comes in between rdtsc and mwaitx, then we oversleep, but by
> at most the time it takes to handle an NMI, and nothing would have
> stopped us from oversleeping that long if an NMI came in right after
> mwaitx returned.

Actually, I'm thinking about an NMI happening after we've issued MWAIT.
NMIs wake it up. So you have the same problem as above:

NMIs will wake MWAIT so you'd need to check how long you've slept and
sleep for the remaining time. I.e., something like that thing from a
couple of mails ago:

delay = usec_to_tsc(delay_usec);

if (delay > ((1 << 32) - 1)) {
mdelay(delay_usec);
return;
}

end = rdtsc() + delay;
while (1) {

monitorx( ...); /* Do we need that here? */
mwaitx(delay);

/* possible wakeups */

now = rdtsc();
if (end <= now)
break;
delay = end - now;
}

Yes, no?

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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