Messages in this thread | | | Subject | Re: [PATCH v3 2/2] x86, cpuid: allow cpuid_read() to schedule | From | Eric Dumazet <> | Date | Fri, 23 Mar 2018 18:01:07 -0700 |
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On 03/23/2018 03:17 PM, H. Peter Anvin wrote: > 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.
Indeed, assuming a daemon can have threads running on all cpus :/
Some environments like to partition cpus for different jobs/containers.
Yes, we can avoid IPI by carefully re-designing these user programs.
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