Re: [PATCH RFC nohz_full 0/7] v2 Provide infrastructure forfull-system idle
From: Paul E. McKenney
Date: Mon Jul 01 2013 - 15:56:30 EST
On Mon, Jul 01, 2013 at 07:43:47PM +0000, Christoph Lameter wrote:
>
On 2013年6月28日, Paul E. McKenney wrote:
>
>
> Unfortunately, timekeeping CPU continues taking scheduling-clock
>
> interrupts even when all other CPUs are completely idle, which is
>
> not so good for energy efficiency and battery lifetime. Clearly, it
>
> would be good to turn off the timekeeping CPU's scheduling-clock tick
>
> when all CPUs are completely idle. This is conceptually simple, but
>
> we also need good performance and scalability on large systems, which
>
> rules out implementations based on frequently updated global counts of
>
> non-idle CPUs as well as implementations that frequently scan all CPUs.
>
> Nevertheless, we need a single global indicator in order to keep the
>
> overhead of checking acceptably low.
>
>
Can we turn off timekeeping when no cpu needs time in adaptive mode?
>
Setting breakpoints in the VDSO could force timekeeping on again whenever
>
something needs time. Would this not be simpler?
Might be. But what causes the breakpoints to be set on a system where
there is one CPU-bound nohz_full user-mode task with all other CPUs idle?
Or are you suggesting taking a breakpoint trap on each timekeeping access
to VDSO?
Thanx, Paul
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