Re: [PATCH 1/1] perf: Add CPU hotplug support for events

[Raghavendra Rao Ananta]

On 02/16/2018 12:39 PM, Peter Zijlstra wrote:
> On Fri, Feb 16, 2018 at 10:06:29AM -0800, Raghavendra Rao Ananta wrote:
> > > No this is absolutely disguisting. You can simply keep the events in the
> > > dead CPU's context. It's really not that hard.
> > Keeping the events in the dead CPU's context was also an idea that we had.
> > However, detaching that event from the PMU when the CPU is offline would be
> > a pain. Consider the scenario in which an event is about to be destroyed
> > when the CPU is offline (yet still attached to the CPU). During it's
> > destruction, a cross-cpu call is made (from perf_remove_from_context()) to
> > the offlined CPU to detach the event from the CPU's PMU. As the CPU is
> > offline, that would not be possible, and again a separate logic has to be
> > written for cleaning up the events whose CPUs are offlined.
>
> That is actually really simple to deal with. The real problems are with
> semantics, is an event enabled when the CPU is dead? Can you
> disable/enable an event on a dead CPU.
>
> The below patch (_completely_ untested) should do most of it, but needs
> help with the details. I suspect we want to allow enable/disable on
> events that are on a dead CPU, and equally I think we want to account
> the time an enabled event spends on a dead CPU to go towards the
> 'enabled' bucket.
I've gone through your diff, and it gave me a hint of similar texture 
what we are trying to do (except for maintaining an offline event list). 
Nevertheless, I tried to test your patch. I created an hw event, and 
tried to offline the CPU in parallel, and I immediately hit a watchdog 
soft lockup bug! Tried the same this by first switching off the CPU 
(without any event created), and I hit into similar issue. I am sure we 
can fix it, but apart from the "why we are doing hotplug?" question, was 
was there specifically any issue with our patch?

> > > Also, you _still_ don't explain why you care about dead CPUs.
I wanted to understand, if we no longer care about hotplugging of CPUs, 
then why do we still have exported symbols such as cpu_up() and 
cpu_down()? Moreover, we also have the hotplug interface exposed to 
users-space as well (through sysfs). As long as these interfaces exist, 
there's always a potential chance of bringing the CPU up/down. Can you 
please clear this thing up for me?

-- Raghavendra

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