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Date: Tue, 27 Mar 2012 11:08:05 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
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Subject: Re: [PATCH 08/32] nohz: Try not to give the timekeeping duty to an
 adaptive tickless cpu
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On Tue, 27 Mar 2012, Frederic Weisbecker wrote:

> > Any way to manually specify which cpu? We f.e. always "sacrifice" cpu 0
> > for OS activities. We would like to have all Os processing things
> > restricted to cpu 0 so that the rest of the processors do not experience
> > the OS noise.
>
> Somebody tries to do this: https://lkml.org/lkml/2011/11/8/346
>
> But in the case of nohz cpusets there is a problem to solve:
>
> What if every CPUs are tickless (idle or busy), who must take
> the timekeeping duty? Should we pick one of the busy CPUs? Or
> keep one CPU with the tick even if it's idle? How do we choose
> this CPU?

Then its the users fault because he specified the processor to use. There
is no picking if its manually assigned.

> May be we need to define another flag on cpusets to assign the
> timekeeping duty to any CPU on a flagged set. This way we can
> force that duty to the CPU(s) we want.

I wish you would disentangle the nohz work from the cpusets. Cpusets is
aged and being replaced by cgroups. And the cgroup work is something that
is not suitable for many loads given the VM overhead added.