linux-kernel - Re: [PATCH] cpusets: Make cpus_allowed and mems

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Message-ID: <20141009083259.GQ10832@worktop.programming.kicks-ass.net>
Date:	Thu, 9 Oct 2014 10:32:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	lizefan@...wei.com, anton@...ba.org, tj@...nel.org,
	svaidy@...ux.vnet.ibm.com, rjw@...ysocki.net,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpusets: Make cpus_allowed and mems_allowed masks
 hotplug invariant

On Thu, Oct 09, 2014 at 01:50:52PM +0530, Preeti U Murthy wrote:
> >> SMT 8 on Power8 can help/hinder workloads. Hence we dynamically switch
> >> the modes at runtime.
> > 
> > That's just a horrible piece of crap hack and you deserve any and all
> > pain you get from doing it.
> > 
> > Randomly removing/adding cpus like that is horrible and makes a mockery
> > of all the affinity interfaces we have.
> 
> We observed this on ubuntu kernel, in which systemd explicitly mounts
> cgroup controllers under a child cgroup identified by the user pid.
> Since we had not observed this additional cgroup being added under the
> hood, it came as a surprise to us that cgroup/cpuset handling in the
> kernel should indeed kick in.
> 
> At best we expect hotplug to be handled well if the users have not
> explicitly configured cpusets, hence implicitly specifying that task
> affinity is for all online cpus. This is indeed the case today, so that
> is good.
> 
> However what remains to be answered is that the V2 of cgroup design -
> the default hierarchy, tracks hotplug operations for children cgroups as
> well. Tejun, Li, will not the concerns that Peter raised above hold for
> the default hierarchy as well?

None of this addresses the piece of crap thing you did with power8. You
cannot just make CPUs go away at random.
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