Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi | Date | Tue, 05 Jun 2012 10:10:33 +0930 |
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On Mon, 04 Jun 2012 22:33:21 +0200, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, 2012-06-04 at 22:11 +0200, Thomas Gleixner wrote: > > > I understand what you are trying to do, though I completely disagree > > with the solution. > > > > The main problem of the current hotplug code is that it is an all or > > nothing approach. You have to tear down the whole thing completely > > instead of just taking it out of the usable set of cpus. > > > > I'm working on a proper state machine driven online/offline sequence, > > where you can put the cpu into an intermediate state which avoids > > bringing it down completely. This is enough to get the full > > powersaving benefits w/o having to go through all the synchronization > > states of a full online/offline. That will shorten the onlining time > > of an previously offlined cpu to almost nothing. > > > > I really want to avoid adding more bandaids to the hotplug code before > > we have sorted out the existing horror. > > Its far worse.. you shouldn't _ever_ care about hotplug latency unless > you've got absolutely braindead hardware. We all now ARM has been > particularly creative here, but is Intel now trying to trump ARM at > stupid?
I disagree. Deactivating a cpu for power saving is halfway to hotplug anyway. I'd rather unify the two cases, where we can specify how dead a CPU should be, than have individual archs and boards do random hacks.
It also gives us a great excuse to audit and neaten various of the hotplug cpu callbacks; most of the ones I've looked at have been racy :(
The ones which simply want to keep per-cpu stats can be given a nice helper with two simple callbacks: one to empty stats for a going-away cpu, and (maybe) one to restore them.
The per-cpu kthreads should no longer get torn down and recreated, and doing it via a separate notifier function is ugly and error-prone. My plan is a "bool kthread_cpu_going(void)" and then a "void kthread_cpu_can_go(void)", so kthreads can do:
if (kthread_cpu_going()) { /* Do any cleanup we need. */ ...
/* This returns when CPU comes back. */ kthread_cpu_can_go(); }
Yeah, we should probably have the kthread exit inside kthread_cpu_can_go() if they stop the kthread, but that's a detail.
Cheers, Rusty.
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