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

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Date:	Sat, 24 Oct 2015 08:50:54 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer.

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
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