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

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Message-ID: <20150521170855.GB23652@pd.tnic>
Date:	Thu, 21 May 2015 19:08:55 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Huang Rui <ray.huang@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Ingo Molnar <mingo@...nel.org>, Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>, "Li, Tony" <Tony.Li@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a
 configurable timer

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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