linux-kernel - Re: [PATCH v2] PM / Sleep: Timer quiesce in freeze state

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Date:	Thu, 13 Nov 2014 10:19:20 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Li, Aubrey" <aubrey.li@...ux.intel.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Brown, Len" <len.brown@...el.com>,
	"alan@...ux.intel.com" <alan@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	"linux-pm@...r.kernel.org >> Linux PM list" 
	<linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2] PM / Sleep: Timer quiesce in freeze state

On Thu, 13 Nov 2014, Li, Aubrey wrote:
> On 2014/11/13 9:37, Peter Zijlstra wrote:
> > On Wed, Nov 12, 2014 at 10:09:47PM +0100, Thomas Gleixner wrote:
> >> On Thu, 30 Oct 2014, Li, Aubrey wrote:
> >>
> >>> Freeze is a general power saving state that processes are frozen, devices
> >>> are suspended and CPUs are in idle state. However, when the system enters
> >>> freeze state, there are a few timers keep ticking and hence consumes more
> >>> power unnecessarily. The observed timer events in freeze state are:
> >>> - tick_sched_timer
> >>> - watchdog lockup detector
> >>> - realtime scheduler period timer
> >>>
> >>> The system power consumption in freeze state will be reduced significantly
> >>> if we quiesce these timers.
> >>
> >> So the obvious question is why dont we quiesce these timers by telling
> >> the subsystems which manage these timers to shut them down?
> >>
> >> I really want a proper answer for this in the first place, but let me
> >> look at the proposed "solution" as well.
> > 
> > Two arguments here:
> > 
> >  1) the current suspend modes don't care, so if this suspend mode starts
> >  to care, its likely to 'break' in the future simply because people
> >  never cared about timers.
> > 
> >  2) there could be userland timers, userland is frozen but they'll still
> >  have their timers set and those can and will fire.
> > 
> > But sure, we can add suspend notifiers to stuff to shut down timers; I
> > should have a patch for at least one of the offenders somewhere. But I
> > really think that we should not be looking at the individual timers for
> > this, none of the other suspend modes care about active timers.
> > 
> >> But before we do that we want a proper explanation why the interrupt
> >> fires at all. The lack of explanation cleary documents that this is a
> >> 'hacked it into submission' approach.
> > 
> >>From what I remember its the waking interrupt that ends up in the
> > timekeeping code, Li should have a backtrace somwhere.
> 
> There are two race conditions:
> 
> The first one occurs after the interrupt is disabled and before we
> suspend lapic. In this time slot, if apic timer interrupt occurs, the
> interrupt is pending there because the interrupt is disabled. Then we
> suspend timekeeping, and then we enter idle and exit idle with interrupt
> re-enabled, the timer interrupt is handled with timekeeping is
> suspended.
> 
> The other occurs after timekeeping_suspended = 1 and before we suspend
> lapic. In this time slot, if apic timer interrupt occurs, we invoke the
> timer interrupt while timekeeping is suspended as above.

And that race exists for every implementation and is not at all apic
timer specific. So we fix it at the core and not at some random place
in the architecture code.

Thanks,

	tglx
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