Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let thetimekeeper sleep

[Andy Lutomirski]

On Wed, Dec 18, 2013 at 1:49 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Dec 18, 2013 at 01:29:53PM -0800, Andy Lutomirski wrote:
>> On 12/18/2013 09:43 AM, Frederic Weisbecker wrote:
>> > On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote:
>> >> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote:
>> >>> So this is what this series brings, more details following:
>> >>>
>> >>> * Some code, naming and whitespace cleanups
>> >>>
>> >>> * Allow all CPUs outside the nohz_full range to handle the timekeeping
>> >>>   duty, not just CPU 0. Balancing the timekeeping duty should improve
>> >>>   powersavings.
>> >>
>> >> If the system just has one nohz_full cpu running, it will need another
>> >> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living.
>> >> From powersaving POV, that is not good compare to normal nohz idle.
>> >
>> > Sure, but everything has a tradeoff :)
>> >
>> > We could theoretically run with the timekeeper purely idle if the other
>> > CPU in full dynticks mode runs in userspace for a long while and seldom
>> > do syscalls and faults. Timekeeping could be updated on kernel/user
>> > boundaries in this case without much impact on performances.
>> >
>> > But then there is one strict condition for that: it can't read the timeofday
>> > through the vdso but only through a syscall.
>>
>> Where's your ambition? :)
>>
>> If the vdso timing functions could see that it's been too long since a
>> real timekeeping update, they could fall back to a syscall.  Otherwise,
>> they could using rdtsc or whatever is in use.
>
> One objection to that approach in the past has been that it injects
> avoidable latency into the worker CPUs.  I suppose that you could argue
> that the cache misses due to a timekeeping-CPU update are not free, but
> then again, the syscall is likely to also incur a few cache misses as
> well.
>
> I bet that the timekeeping-CPU approach wins, but it would be cool to
> see you prove me wrong.

There's already some (very vague) discussion about having a scheduled
time at which the clock frequency and/or offset will change, and this
wouldn't be a huge departure from that.  The goal there is to avoid
waiting for timekeeping if vclock_gettime runs concurrently with an
update, but the same approach could apply here (albeit with one extra
branch).

Anyway, syscalls aren't *that* expensive.

Alternatively, couldn't workloads like this just turn off NTP?

--Andy
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