Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer.

[Yunhong Jiang]

On Sat, Oct 24, 2015 at 08:50:54AM +0530, Viresh Kumar wrote:
> On 23-10-15, 15:10, Yunhong Jiang wrote:
> > I got this impression from Frederic's comments on 
> > http://marc.info/?l=linux-kernel&m=139048415303210&w=2, "So you simply rely 
> > on the next tick to see the new timer. This should work with 
> > CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be 
> > running without the tick".
> > Per my understanding of this comment, it means we can rely on the next tick 
> > for CONFIG_NO_HZ_IDLE, which means it's sure a tick will happen for 
> > CONFIG_NO_HZ_IDLE, am I right?
> 
> Yeah, the CPU wouldn't like in idle for ever but the time is not known
> and it can be really really long.
> 
> > Hmm, per http://lxr.free-electrons.com/source/include/linux/timer.h#L51, the 
> > deferreable timer will be serviced when the CPU eventually wakes up "with a 
> > subsequent non-deferrable timer".
> 
> It will be an IPI mostly..
> 
> > If there is no non-deferrable timer, based 
> > on Frederic's comments, we in fact depends on next tick.
> 
> So, the cpu will wake up when it receives an IPI. The first thing we
> do then is to restart the tick and we will then service all the
> pending deferred timers.
> 
> > My confusion is, why we are sure there is next tick on CONFIG_NO_HZ_IDLE 
> > idle processor to wake it up. If there is no tick, and no other timer, will 
> > the timer get no chance to be waken up at all? I don't think "deferred for 
> > ever" is deferreable.
> 
> There are many kind of works we may want to do. If its really
> important to be done earlier, then it should be serviced with a timer.
> 
> deferred timers are better used for activities, which are irrelevant
> once the CPU is idle. One case is doing some per-cpu load tracking for
> cpufreq governors or the work that vmstat does.
> 
> Even if the CPU wakes up after few hours (hypothetically), it
> shouldn't matter.

Viresh, thanks for the clarification.

So seems the original patch is correct to wakeup the full dyntick CPU even 
for deferred timer. Thomas/Fred, your idea?

Thanks
--jyh
> 
> -- 
> viresh
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