Re: Is it ok for deferrable timer wakeup the idle cpu?

From: Viresh Kumar
Date: Thu Jan 23 2014 - 00:52:39 EST

On 23 January 2014 11:11, Lei Wen <adrian.wenl@xxxxxxxxx> wrote:
> On Wed, Jan 22, 2014 at 10:07 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> On Wed, 22 Jan 2014, Lei Wen wrote:
>>> Recently I want to do the experiment for cpu isolation over 3.10 kernel.
>>> But I find the isolated one is periodically waken up by IPI interrupt.
>>> By checking the trace, I find those IPI is generated by add_timer_on,
>>> which would calls wake_up_nohz_cpu, and wake up the already idle cpu.
>>> With further checking, I find this timer is added by on_demand governor of
>>> cpufreq. It would periodically check each cores' state.
>>> The problem I see here is cpufreq_governor using INIT_DEFERRABLE_WORK
>>> as the tool, while timer is made as deferrable anyway.
>>> And what is more that cpufreq checking is very frequent. In my case, the
>>> isolated cpu is wakenup by IPI every 5ms.
>>> So why kernel need to wake the remote processor when mount the deferrable
>>> timer? As per my understanding, we'd better keep cpu as idle when use
>>> the deferrable timer.
>> Indeed, we can avoid the wakeup of the remote cpu when the timer is
>> deferrable.
> Glad to hear that we could fix this unwanted wakeup.
> Do you have related patches already?
>> Though you really want to figure out why the cpufreq governor is
>> arming timers on other cores every 5ms. That smells like an utterly
>> stupid approach.
> Not sure why cpufreq choose such frequent profiling over each cpu.
> As my understanding, since kernel is smp, launching profiler over one cpu
> would be enough...

Hi Guys,

So the first question is why cpufreq needs it and is it really stupid?
Yes, it is stupid but that's how its implemented since a long time. It does
so to get data about the load on CPUs, so that freq can be scaled up/down.

Though there is a solution in discussion currently, which will take
inputs from scheduler and so these background timers would go away.
But we need to wait until that time.

Now, why do we need that for every cpu, while that for a single cpu might
be enough? The answer is cpuidle here: What if the cpu responsible for
running timer goes to sleep? Who will evaluate the load then? And if we
make this timer run on one cpu in non-deferrable mode then that cpu
would be waken up again and again from idle. So, it was decided to have
a per-cpu deferrable timer. Though to improve efficiency, once it is fired
on any cpu, timer for all other CPUs are rescheduled, so that they don't
fire before 5ms (sampling time)..

I think below diff might get this fixed for you, though I am not sure if it
breaks something else. Probably Thomas/Frederic can answer here.
If this looks fine I will send it formally again:

diff --git a/kernel/timer.c b/kernel/timer.c
index accfd24..3a2c7fa 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -940,7 +940,8 @@ void add_timer_on(struct timer_list *timer, int cpu)
* makes sure that a CPU on the way to stop its tick can not
* evaluate the timer wheel.
- wake_up_nohz_cpu(cpu);
+ if (!tbase_get_deferrable(timer->base))
+ wake_up_nohz_cpu(cpu);
spin_unlock_irqrestore(&base->lock, flags);
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