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Commit 07cde313 authored by Eric Dumazet's avatar Eric Dumazet Committed by Thomas Gleixner
Browse files

x86/msr: Allow rdmsr_safe_on_cpu() to schedule 



High latencies can be observed caused by a daemon periodically reading
various MSR on all cpus. On KASAN enabled kernels ~10ms latencies can be
observed simply reading one MSR. 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.

All usage sites are in preemptible context, convert rdmsr_safe_on_cpu() to
use a completion instead of busy polling.

Overall daemon cpu usage was reduced by 35 %, and latencies caused by
msr_read() disappeared.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarIngo 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/20180323215818.127774-1-edumazet@google.com
parent 13cc36d7

arch/x86/lib/msr-smp.c

+24 −8
Original line number Diff line number Diff line
@@ -2,6 +2,7 @@ 
#include <linux/export.h>

#include <linux/preempt.h>

#include <linux/smp.h>

#include <linux/completion.h>

#include <asm/msr.h>



static void __rdmsr_on_cpu(void *info)
@@ -143,13 +144,19 @@ void wrmsr_on_cpus(const struct cpumask *mask, u32 msr_no, struct msr *msrs) 
}

EXPORT_SYMBOL(wrmsr_on_cpus);



struct msr_info_completion {

	struct msr_info		msr;

	struct completion	done;

};



/* These "safe" variants are slower and should be used when the target MSR

   may not actually exist. */

static void __rdmsr_safe_on_cpu(void *info)

{

	struct msr_info *rv = info;

	struct msr_info_completion *rv = info;



	rv->err = rdmsr_safe(rv->msr_no, &rv->reg.l, &rv->reg.h);

	rv->msr.err = rdmsr_safe(rv->msr.msr_no, &rv->msr.reg.l, &rv->msr.reg.h);

	complete(&rv->done);

}



static void __wrmsr_safe_on_cpu(void *info)
@@ -161,17 +168,26 @@ static void __wrmsr_safe_on_cpu(void *info) 


int rdmsr_safe_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h)

{

	struct msr_info_completion rv;

	call_single_data_t csd = {

		.func	= __rdmsr_safe_on_cpu,

		.info	= &rv,

	};

	int err;

	struct msr_info rv;



	memset(&rv, 0, sizeof(rv));

	init_completion(&rv.done);

	rv.msr.msr_no = msr_no;



	rv.msr_no = msr_no;

	err = smp_call_function_single(cpu, __rdmsr_safe_on_cpu, &rv, 1);

	*l = rv.reg.l;

	*h = rv.reg.h;

	err = smp_call_function_single_async(cpu, &csd);

	if (!err) {

		wait_for_completion(&rv.done);

		err = rv.msr.err;

	}

	*l = rv.msr.reg.l;

	*h = rv.msr.reg.h;



	return err ? err : rv.err;

	return err;

}

EXPORT_SYMBOL(rdmsr_safe_on_cpu);