linux-kernel - Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote wakeups

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Message-ID: <1307019866.2497.675.camel@laptop>
Date:	Thu, 02 Jun 2011 15:04:26 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Borislav Petkov <bp@...64.org>, Borislav Petkov <bp@...en8.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"markus@...ppelsdorf.de" <markus@...ppelsdorf.de>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remote
 wakeups

On Thu, 2011-06-02 at 15:52 +0800, Yong Zhang wrote:
> In sched_clock_local(), clock is calculated around ->tick_gtod even if
> that ->tick_gtod is stale for long time because we stays in idle state. 
> You know ->tick_gtod is only updated in sched_clock_tick();

(well, no, there's idle callbacks as you said below)

> IOW, when a cpu goes out of idle, sched_clock_tick() is called from
> tick_nohz_stop_idle() which is later than interrupt.

Gah, that would be awefull and mean wakeups from interrupts were already
borken. /me goes look at code.

irq_enter() -> tick_check_idle() -> tick_check_nohz() ->
tick_nohz_stop_idle() -> sched_clock_idle_wakeup_event()

should update the thing before we run any isrs, right?

> So if we have any site which call sched_clock() in interrupt on an
> idle cpu, it could get incorrect clock.
> 
> I'm not sure how to teach sched_clock() about this special case, Peter?

isn't anything to teach afaict.

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