Commit 67bbd7a8 authored by Eric Dumazet's avatar Eric Dumazet Committed by Thomas Gleixner
Browse files

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: 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/20180323215818.127774-2-edumazet@google.com
parent 07cde313

arch/x86/kernel/cpuid.c

+26 −8
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ 
#include <linux/notifier.h>

#include <linux/uaccess.h>

#include <linux/gfp.h>

#include <linux/completion.h>



#include <asm/processor.h>

#include <asm/msr.h>
@@ -47,19 +48,27 @@ 
static struct class *cpuid_class;

static enum cpuhp_state cpuhp_cpuid_state;



struct cpuid_regs_done {

	struct cpuid_regs regs;

	struct completion done;

};



static void cpuid_smp_cpuid(void *cmd_block)

{

	struct cpuid_regs *cmd = (struct cpuid_regs *)cmd_block;

	struct cpuid_regs_done *cmd = cmd_block;



	cpuid_count(cmd->regs.eax, cmd->regs.ecx,

		    &cmd->regs.eax, &cmd->regs.ebx,

		    &cmd->regs.ecx, &cmd->regs.edx);



	cpuid_count(cmd->eax, cmd->ecx,

		    &cmd->eax, &cmd->ebx, &cmd->ecx, &cmd->edx);

	complete(&cmd->done);

}



static ssize_t cpuid_read(struct file *file, char __user *buf,

			  size_t count, loff_t *ppos)

{

	char __user *tmp = buf;

	struct cpuid_regs cmd;

	struct cpuid_regs_done cmd;

	int cpu = iminor(file_inode(file));

	u64 pos = *ppos;

	ssize_t bytes = 0;
@@ -68,19 +77,28 @@ static ssize_t cpuid_read(struct file *file, char __user *buf, 
	if (count % 16)

		return -EINVAL;	/* Invalid chunk size */



	init_completion(&cmd.done);

	for (; count; count -= 16) {

		cmd.eax = pos;

		cmd.ecx = pos >> 32;

		err = smp_call_function_single(cpu, cpuid_smp_cpuid, &cmd, 1);

		call_single_data_t csd = {

			.func = cpuid_smp_cpuid,

			.info = &cmd,

		};



		cmd.regs.eax = pos;

		cmd.regs.ecx = pos >> 32;



		err = smp_call_function_single_async(cpu, &csd);

		if (err)

			break;

		if (copy_to_user(tmp, &cmd, 16)) {

		wait_for_completion(&cmd.done);

		if (copy_to_user(tmp, &cmd.regs, 16)) {

			err = -EFAULT;

			break;

		}

		tmp += 16;

		bytes += 16;

		*ppos = ++pos;

		reinit_completion(&cmd.done);

	}



	return bytes ? bytes : err;

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