Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Srivatsa S. Bhat]

On 10/10/2013 08:56 PM, Oleg Nesterov wrote:
> On 10/10, Ingo Molnar wrote:
>>
>> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>>> But the thing is; our sense of NR_CPUS has shifted, where it used to be
>>> ok to do something like:
>>>
>>>   for_each_cpu()
>>>
>>> With preemption disabled; it gets to be less and less sane to do so,
>>> simply because 'common' hardware has 256+ CPUs these days. If we cannot
>>> rely on preempt disable to exclude hotplug, we must use
>>> get_online_cpus(), but get_online_cpus() is global state and thus cannot
>>> be used at any sort of frequency.
>>
>> So ... why not make it _really_ cheap, i.e. the read lock costing nothing,
>> and tie CPU hotplug to freezing all tasks in the system?
>>
>> Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a
>> system, I don't understand how we tolerate _any_ overhead from this utter
>> slowpath.
> 
> Well, iirc Srivatsa (cc'ed) pointed out that some systems do cpu_down/up
> quite often to save the power.
> 

Yes, I've heard of such systems and so I might have brought them up during
discussions about CPU hotplug. But unfortunately, I have been misquoted quite
often, leading to the wrong impression that I have such a usecase or that I
recommend/support using CPU hotplug for power management. So let me clarify
that part, while I have the chance. (And I don't blame anyone for that. I
work on power-management related areas, and I've worked on improving/optimizing
CPU hotplug; so its pretty natural to make a connection between the two
and assume that I tried to optimize CPU hotplug keeping power management
in mind. But that's not the case, as I explain below.)

I started out trying to make suspend/resume more reliable, scalable and fast.
And suspend/resume uses CPU hotplug underneath and that's a pretty valid usecase.
So with that, I started looking at CPU hotplug and soon realized the mess it
had become. So I started working on cleaning up that mess, like rethinking the
whole notifier scheme[1], and removing the ridiculous stop_machine() from the
cpu_down path[2] etc. But the intention behind all this work was just to make
CPU hotplug cleaner/saner/bug-free and possibly speed up suspend/resume. IOW,
I didn't have any explicit intention to make it easier for people to use it
for power management, although I understood that some of this work might
help those poor souls who don't have any other choice, for whatever reason.
And fortunately, (IIUC) the number of systems/people relying on CPU hotplug for
power management has reduced quite a bit in the recent times, which is a very
good thing.

So, to reiterate, I totally agree that power-aware scheduler is the right way
to do CPU power management; CPU hotplug is simply not the tool to use for that.
No question about that. Also, system shutdown used to depend on CPU hotplug to
disable the non-boot CPUs, but we don't do that any more after commit cf7df378a,
which is a very welcome change. And in future if we can somehow do suspend/resume
without using CPU hotplug, that would be absolutely wonderful as well. (There
have been discussions in the past around this, but nobody has a solution yet).
The other valid usecases that I can think of, for using CPU hotplug, is for RAS
reasons and for DLPAR (Dynamic Logical Partitioning) operations on powerpc, both
of which are not performance-sensitive, AFAIK.


[1]. Reverse invocation of CPU hotplug notifiers
     http://lwn.net/Articles/508072/

[2]. Stop-machine()-free CPU hotplug
     http://lwn.net/Articles/538819/ (v6)
     http://lwn.net/Articles/556727/

Regards,
Srivatsa S. Bhat

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