Re: [PATCH V2] cpuset: Add knob to make allowed masks hotplug invariant on legacy hierarchy

From: Serge E. Hallyn
Date: Wed Apr 15 2015 - 11:03:19 EST

On Wed, Apr 15, 2015 at 05:10:49PM +0530, Preeti U Murthy wrote:
> On 04/13/2015 08:13 PM, Peter Zijlstra wrote:
> > On Mon, Apr 13, 2015 at 05:46:37PM +0530, Preeti U Murthy wrote:
> >> On 04/13/2015 12:31 PM, Peter Zijlstra wrote:
> >
> >>> Would it not make sense to make that a mount option and limit the amount
> >>> of semantic variants of cpusets?
> >>
> >> I spent some time analyzing if this would be a better option than the
> >> sysfs knob and I think not for the following reasons:
> >>
> >> 1. Mount options tend to be generic across the controllers of a cgroup.
> >> But use case addressed by this patch is specific to the cpuset controller.
> >
> > Surely we can get around that somehow.
> >
> >> 2. The behavior that this patch is trying to bring about is not a
> >> drastic one to call for a mount option equivalent to the __SANE_BEHAVIOR
> >> one that existed earlier. This option was used to switch the legacy
> >> design to the default one.
> >>
> >> However this patch is not *wholly* mimicking the default hierarchy
> >> behavior. The behavior when cpusets become empty is left untouched for
> >> instance. The patch borrows one of the behaviors from the default
> >> hierarchy only and hence just not justify the use of a mount flag.
> >
> > So the 'problem' I have is that you introduce a 3rd semantic for the
> > cpuset thing.
> >
> > You also do not answer if you can live with the default hierarchy
> > behaviour, only that your patch mimicks a subset of it.
> >
> > Why not all of it?
> This was assuming that the existing software will break if we mimick the
> entire design given that we were informed that it does not work well
> with the default hierarchy. But I think now, that its worth finding out
> why if so and switch over to the new design, atleast for cpusets.

Peter, is the question "why can't we just use the unified hierarchy for
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