Re: [PATCH v3 2/2] x86, cpuid: allow cpuid_read() to schedule

[H. Peter Anvin]

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