Re: [PATCH v3 5/5] cpusets, suspend: Save and restore cpusets during suspend/resume

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 05/16/2012 02:54 AM, David Rientjes wrote:

> On Tue, 15 May 2012, Peter Zijlstra wrote:
> 
>>> Why can't we leave cpuset.cpus unaltered for all cpusets during suspend?  
>>
>> We can, that's what Srivatsa is going to make. The only thing I object
>> to is the !suspend hotplug case.
>>
> 
> Srivatsa is going to do that in another patchset in addition to this one?


Nope. This v3 itself is the full implementation.

> We shouldn't need to store any old cpumask at all, just allow cpuset.cpus 
> to be a superset of online cpus during s/r and don't touch cpusets at all 
> since the cpus, as you said, are guaranteed to come back.
> 


Oh, I am really sorry to say this, but this method has got 'history' ;-)
(Argh, I really should have put pointers to v1 and v2 in patch 0/5...)

What you are suggesting was precisely the v1 of this patchset, which went
upstream as commit 8f2f748b06562 (CPU hotplug, cpusets, suspend: Don't touch
cpusets during suspend/resume).

It got reverted due to a nasty suspend hang in some corner case, where the
sched domains not being up-to-date got the scheduler confused.
Here is the thread with that discussion:
http://thread.gmane.org/gmane.linux.kernel/1262802/focus=1286289

As Peter suggested, I'll try to fix the issues at the 2 places that I found
where the scheduler gets confused despite the cpu_active mask being up-to-date.

But, I really want to avoid that scheduler fix and this cpuset fix from
being tied together, for the fear that until we root-cause and fix all
scheduler bugs related to cpu_active mask, we can never get cpusets fixed
once and for all for suspend/resume. So, this patchset does an explicit
save and restore to be sure, and so that we don't depend on some other/unknown
factors to make this work reliably.

Regards,
Srivatsa S. Bhat