Messages in this thread | | | Date | Thu, 8 Oct 2015 18:08:45 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to rely on stopper->enabled |
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On 10/08, Oleg Nesterov wrote: > > On 10/08, Peter Zijlstra wrote: > > > > On Thu, Oct 08, 2015 at 04:51:36PM +0200, Oleg Nesterov wrote: > > > @@ -261,12 +276,8 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void * > > > set_state(&msdata, MULTI_STOP_PREPARE); > > > > > > /* > > > + * We do not want to migrate to inactive CPU. FIXME: move this > > > + * into the caller. > > > */ > > > if (!cpu_active(cpu1) || !cpu_active(cpu2)) { > > > preempt_enable(); > > > > So we cannot move that into the caller.. > > Why? > > > because this function sleeps > > with wait_for_completion(). > > > > Or rather, it would force the caller to use get_online_cpus(), which we > > worked really hard to avoid. > > Aaah wait. Sorry for confusion! > > I meant "move this into the callback, migrate_swap_stop()".
Forgot to mention... And note that both these cpu_active() are obviously racy, CPU_DOWN_PREPARE can make it inactive right after the check.
> > Also, I think we still want the patch I proposed which ensures the > > stopper thread is active 'early', because the load balancer pretty much > > Perhaps. Although I do not really understand why it is important. > I mean, either way we unpark it at CPU_ONLINE stage, just > sched_cpu_active() has a higher priority. > > But this is off-topic in a sense that the main point of this patch > is that stop_two_cpus() no longer needs to abuse cpu_active() checks > to avoid the race with cpu_up/down, we can simply rely on ->enabled. > > And again, we need to take both locks to remove "lglock stop_cpus_lock". > > So I think your change can be applied after this series too. Or I missed > something?
So I think this series makes sense anyway and hopefully should fix the problem with or without your change.
Oleg.
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