Messages in this thread | | | Date | Sat, 15 May 2021 01:28:39 +0200 | From | Peter Zijlstra <> | Subject | Re: [patch 7/8] hrtimer: Avoid unnecessary SMP function calls in clock_was_set() |
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On Fri, May 14, 2021 at 08:52:33PM +0200, Thomas Gleixner wrote: > On Thu, May 13 2021 at 16:59, Peter Zijlstra wrote: > > On Tue, Apr 27, 2021 at 10:25:44AM +0200, Thomas Gleixner wrote: > >> - /* Retrigger the CPU local events everywhere */ > >> - on_each_cpu(retrigger_next_event, NULL, 1); > >> + if (!zalloc_cpumask_var(&mask, GFP_KERNEL)) { > >> + on_each_cpu(retrigger_next_event, NULL, 1); > > > > This will violate NOHZ_FULL; > > Only if that allocation fails.
Right, which should be near to never I suppose.
> Aside of that any CPU which has an affected timer will get notified even > on NOHZ_FULL.
Right; but if it's properly NOHZ_FULL -- the kind that wanted a signal on any entry into the kernel -- when it won't have timers and this IPI will trigger the signal and kill the program.
But yeah, you're right, that's not very likely.
> >> + preempt_disable(); > >> + smp_call_function_many(mask, retrigger_next_event, NULL, 1); > > > > The sane option is: > > > > smp_call_function_many_cond(cpu_online_mask, retrigger_next_event, > > NULL, SCF_WAIT, update_needs_ipi); > > > > Which does all of the above, but better. > > With the difference that the for_each_cpu() loop runs with preemption > disabled, while with this approach preemption is only disabled accross > the function call.
Yeah, I'd forgotten that... I might put looking at that on the todo list somewhere :/
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