Messages in this thread |  | | Date | Tue, 17 Dec 2019 21:23:52 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] smp: Allow smp_call_function_single_async() to insert locked csd |
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On Tue, Dec 17, 2019 at 10:31:28AM -0500, Peter Xu wrote: > On Tue, Dec 17, 2019 at 10:51:56AM +0100, Peter Zijlstra wrote: > > On Mon, Dec 16, 2019 at 03:58:33PM -0500, Peter Xu wrote: > > > On Mon, Dec 16, 2019 at 09:37:05PM +0100, Peter Zijlstra wrote: > > > > On Wed, Dec 11, 2019 at 11:29:25AM -0500, Peter Xu wrote: > > > > > > > (3) Others: > > > > > > > > > > *** arch/mips/kernel/process.c: > > > > > raise_backtrace[713] smp_call_function_single_async(cpu, csd); > > > > > > > > per-cpu csd data, seems perfectly fine usage. > > > > > > I'm not sure whether I get the point, I just feel like it could still > > > trigger as long as we do it super fast, before IPI handled, > > > disregarding whether it's per-cpu csd or not. > > > > No, I wasn't paying attention last night. I'm thinking this one might > > maybe be in 1). It does the state check using that bitmap. > > Indeed. Though I'm not very certain to change this one too, since I'm > not sure whether that pr_warn is really intended: > > if (cpumask_test_and_set_cpu(cpu, &backtrace_csd_busy)) { > pr_warn("Unable to send backtrace IPI to CPU%u - perhaps it hung?\n", > cpu); > continue; > } > > I mean, that should depend on if it can really hang somehow (or it's > the same issue as what we're trying to fix)... If it won't hang, then > it should be safe I think, and this pr_warn could be helpless after all.
Yeah, leave it.
> > 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().
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