Messages in this thread | | | Subject | Re: [PATCH 1/1] perf: Add CPU hotplug support for events | From | Raghavendra Rao Ananta <> | Date | Fri, 16 Feb 2018 17:48:21 -0800 |
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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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