Messages in this thread |  | | Date | Tue, 17 Dec 2019 15:48:39 -0500 | From | Peter Xu <> | Subject | Re: [PATCH] smp: Allow smp_call_function_single_async() to insert locked csd |
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On Tue, Dec 17, 2019 at 09:23:52PM +0100, Peter Zijlstra wrote: > > > I suspect to be nice for virt. Both CPUID and MSR accesses can trap. but > > > now I'm confused, because it is mostly WRMSR that traps. > > > > > > Anyway, see the commit here: 07cde313b2d2 ("x86/msr: Allow rdmsr_safe_on_cpu() to schedule") > > > > Yes that makes sense. Thanks for the pointer. > > > > However, then my next confusion is why they can't provide a common > > solution to the smp code again... I feel like it could be even easier > > (please see below). I'm not very familiar with smp code yet, but if > > it works it should benefit all callers imho. > > Ah, so going to sleep on wait_for_completion() is _much_ more expensive > than a short spin. So it all depends on the expected behaviour of the > IPI I suppose. > > In general we expect these IPIs to be 'quick'. > > Also, as is, you're allowed to use the smp_call_function*() family with > preemption disabled, which pretty much precludes using > wait_for_completion().
Yes you are right, thanks!
Though I feel like previous small patch could still be an small enhancement in that it at least shortened the get_cpu() period of smp_call_function_single() without losing anything - so that could still give the caller a chance to be scheduled out of the spinning at some point when the IPI conditionally takes time to finish, imho.
-- Peter Xu
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