Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Srivatsa S. Bhat]

On 10/01/2013 11:26 PM, Peter Zijlstra wrote:
> On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote:
>> On 10/01, Peter Zijlstra wrote:
>>>
>>> On Tue, Oct 01, 2013 at 10:41:15PM +0530, Srivatsa S. Bhat wrote:
>>>> However, as Oleg said, its definitely worth considering whether this proposed
>>>> change in semantics is going to hurt us in the future. CPU_POST_DEAD has certainly
>>>> proved to be very useful in certain challenging situations (commit 1aee40ac9c
>>>> explains one such example), so IMHO we should be very careful not to undermine
>>>> its utility.
>>>
>>> Urgh.. crazy things. I've always understood POST_DEAD to mean 'will be
>>> called at some time after the unplug' with no further guarantees. And my
>>> patch preserves that.
>>
>> I tend to agree with Srivatsa... Without a strong reason it would be better
>> to preserve the current logic: "some time after" should not be after the
>> next CPU_DOWN/UP*. But I won't argue too much.
> 
> Nah, I think breaking it is the right thing :-)
> 
>> But note that you do not strictly need this change. Just kill cpuhp_waitcount,
>> then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in
>> another thread, this should likely "join" all synchronize_sched's.
> 
> That would still be 4k * sync_sched() == terribly long.
> 
>> Or split cpu_hotplug_begin() into 2 helpers which handle FAST -> SLOW and
>> SLOW -> BLOCK transitions, then move the first "FAST -> SLOW" handler outside
>> of for_each_online_cpu().
> 
> Right, that's more messy but would work if we cannot teach cpufreq (and
> possibly others) to not rely on state you shouldn't rely on anyway.
> 
> I tihnk the only guarnatee POST_DEAD should have is that it should be
> called before UP_PREPARE of the same cpu ;-) Nothing more, nothing less.
> 

Conceptually, that hints at a totally per-cpu implementation of CPU hotplug,
in which what happens to one CPU doesn't affect the others in the hotplug
path.. and yeah, that sounds very tempting! ;-) but I guess that will
need to be preceded by a massive rework of many of the existing hotplug
callbacks ;-)

Regards,
Srivatsa S. Bhat

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