Messages in this thread | | | Subject | Re: [PATCH v3 2/2] x86, cpuid: allow cpuid_read() to schedule | From | "H. Peter Anvin" <> | Date | Fri, 23 Mar 2018 15:17:44 -0700 |
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On 03/23/18 14:58, Eric Dumazet wrote: > I noticed high latencies caused by a daemon periodically reading various > MSR and cpuid on all cpus. KASAN kernels would see ~10ms latencies > simply reading one cpuid. Even without KASAN, sending IPI to CPU > in deep sleep state or blocking hard IRQ in a a long section, > then waiting for the answer can consume hundreds of usec or more. > > Switching to smp_call_function_single_async() and a completion > allows to reschedule and not burn cpu cycles.
That being said, the Right Way for a daemon to read multiple MSRs and CPUIDs on multiple CPUs is to spawn a thread for each CPU and use CPU affinity to lock them down. No IPI is needed to access MSRs on the current CPU, and CPUID doesn't even need kernel entry.
-hpa
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