Date: Mon, 6 Apr 2015 13:47:35 -0400
From: Tejun Heo <tj@kernel.org>
To: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, lizefan@huawei.com, anton@samba.org,
        svaidy@linux.vnet.ibm.com, rjw@rjwysocki.net,
        paulmck@linux.vnet.ibm.com, mingo@kernel.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, bharata@linux.vnet.ibm.com
Subject: Re: [PATCH] cpusets: Make cpus_allowed and mems_allowed masks
 hotplug invariant
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Hello, Preeti.

On Thu, Apr 02, 2015 at 12:26:32PM +0530, Preeti U Murthy wrote:
> By ensuring that the user configured cpusets are untouched, I don't see
> how we affect userspace adversely. The expectation usually is that the
> kernel keeps track of the user configurations. If anything we would be
> fixing an undesired behavior, wouldn't we?

The problem is not really about which behavior is "righter" but rather
it's fairly likely that there are users / tools out there expecting
the current behavior and they wouldn't be too happy to see the
behavior flipping underneath them.

One way forward would be implementing a knob in cpuset which makes it
switch sbetween the old and new behaviors in the legacy hierarchy.
It's yucky but doable if absoluately necessary, but what's the reason
for you not being able to transition to the unified hierarchy (except
for it being under the devel flag but I'm really taking that devel
mask out in the next merge window)?  The default hierarchy can happily
co-exist with legacy hierarchies so you can just move over the cpuset
part to it if you need it.

Thanks.

-- 
tejun
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