From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751271AbdKMBPx (ORCPT ); Sun, 12 Nov 2017 20:15:53 -0500 Received: from cloudserver094114.home.net.pl ([79.96.170.134]:62958 "EHLO cloudserver094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750954AbdKMBPv (ORCPT ); Sun, 12 Nov 2017 20:15:51 -0500 From: "Rafael J. Wysocki" To: x86@kernel.org Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Linus Torvalds , LKML , Linux PM , Len Brown , WANG Chao Subject: [PATCH] x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu() Date: Mon, 13 Nov 2017 02:15:39 +0100 Message-ID: <1830825.NTYOMyctnU@aspire.rjw.lan> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Rafael J. Wysocki Even though aperfmperf_snapshot_khz() caches the samples.khz value to return if called again in a sufficiently short time, its caller, arch_freq_get_on_cpu(), still uses smp_call_function_single() to run it which may allow user space to trigger an IPI storm by reading from the scaling_cur_freq cpufreq sysfs file in a tight loop. To avoid that, move the decision on whether or not to return the cached samples.khz value to arch_freq_get_on_cpu(). Fixes: 4815d3c56d1e (cpufreq: x86: Make scaling_cur_freq behave more as expected) Signed-off-by: Rafael J. Wysocki --- This change was part of commit 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in /proc/cpuinfo), but it was not the reason for the revert and it remains applicable. Thanks, Rafael --- arch/x86/kernel/cpu/Makefile | 2 +- arch/x86/kernel/cpu/aperfmperf.c | 11 +++++++---- arch/x86/kernel/cpu/proc.c | 4 +++- 3 files changed, 11 insertions(+), 6 deletions(-) Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c =================================================================== --- linux-pm.orig/arch/x86/kernel/cpu/aperfmperf.c +++ linux-pm/arch/x86/kernel/cpu/aperfmperf.c @@ -42,10 +42,6 @@ static void aperfmperf_snapshot_khz(void s64 time_delta = ktime_ms_delta(now, s->time); unsigned long flags; - /* Don't bother re-computing within the cache threshold time. */ - if (time_delta < APERFMPERF_CACHE_THRESHOLD_MS) - return; - local_irq_save(flags); rdmsrl(MSR_IA32_APERF, aperf); rdmsrl(MSR_IA32_MPERF, mperf); @@ -74,6 +70,7 @@ static void aperfmperf_snapshot_khz(void unsigned int arch_freq_get_on_cpu(int cpu) { + s64 time_delta; unsigned int khz; if (!cpu_khz) @@ -82,6 +79,12 @@ unsigned int arch_freq_get_on_cpu(int cp if (!static_cpu_has(X86_FEATURE_APERFMPERF)) return 0; + /* Don't bother re-computing within the cache threshold time. */ + time_delta = ktime_ms_delta(ktime_get(), per_cpu(samples.time, cpu)); + khz = per_cpu(samples.khz, cpu); + if (khz && time_delta < APERFMPERF_CACHE_THRESHOLD_MS) + return khz; + smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1); khz = per_cpu(samples.khz, cpu); if (khz)