linux-imx

x86/cpuid: Allow cpuid_read() to schedule

High latencies can be observed caused by a daemon periodically reading
CPUID on all cpus. On KASAN enabled kernels ~10ms latencies can be
observed. Even without KASAN, sending an IPI to a CPU, which is in a deep
sleep state or in a long hard IRQ disabled section, waiting for the answer
can consume hundreds of microseconds.

cpuid_read() is invoked in preemptible context, so it can be converted to
sleep instead of busy wait.

Switching to smp_call_function_single_async() and a completion allows to
reschedule and reduces CPU usage and latencies.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Link: https://lkml.kernel.org/r/20180323215818.127774-2-edumazet@google.com