From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966980AbeCSREX (ORCPT ); Mon, 19 Mar 2018 13:04:23 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:54752 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965575AbeCSRES (ORCPT ); Mon, 19 Mar 2018 13:04:18 -0400 X-Google-Smtp-Source: AG47ELsAsEP8suxWnLxF63EYZFItCC0M+8DqbNVgXZSfmmH2RT2wSAmexoHl1+rI4jf9GsJICHW4Vw== From: jason.vas.dias@gmail.com To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, andi@firstfloor.org Subject: [PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall Date: Mon, 19 Mar 2018 17:04:00 +0000 Message-Id: <1521479041-17030-1-git-send-email-jason.vas.dias@gmail.com> X-Mailer: git-send-email 2.8.2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Resent to address reviewer comments, and allow builds with compilers that support -DRETPOLINE to succeed. Currently, the VDSO does not handle clock_gettime( CLOCK_MONOTONIC_RAW, &ts ) on Intel / AMD - it calls vdso_fallback_gettime() for this clock, which issues a syscall, having an unacceptably high latency (minimum measurable time or time between measurements) of 300-700ns on 2 2.8-3.9ghz Haswell x86_64 Family'_'Model : 06_3C machines under various versions of Linux. Sometimes, particularly when correlating elapsed time to performance counter values, user-space code needs to know elapsed time from the perspective of the CPU no matter how "hot" / fast or "cold" / slow it might be running wrt NTP / PTP "real" time; when code needs this, the latencies associated with a syscall are often unacceptably high. I reported this as Bug #198161 : 'https://bugzilla.kernel.org/show_bug.cgi?id=198961' and in previous posts with subjects matching 'CLOCK_MONOTONIC_RAW' . This patch handles CLOCK_MONOTONIC_RAW clock_gettime() in the VDSO , by exporting the raw clock calibration, last cycles, last xtime_nsec, and last raw_sec value in the vsyscall_gtod_data during vsyscall_update() . Now the new do_monotonic_raw() function in the vDSO has a latency of @ 20ns on average, and the test program: tools/testing/selftest/timers/inconsistency-check.c succeeds with arguments: '-c 4 -t 120' or any arbitrary -t value. The patch is against Linus' latest 4.16-rc5 tree, current HEAD of : git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git . This patch affects only files: arch/x86/include/asm/vgtod.h arch/x86/entry/vdso/vclock_gettime.c arch/x86/entry/vsyscall/vsyscall_gtod.c Patches for kernels 3.10.0-21 and 4.9.65-rt23 (ARM) are attached to bug #198161, as is the test program, timer_latency.c, to demonstrate the problem. Before the patch a latency of 200-1000ns was measured for clock_gettime(CLOCK_MONOTONIC_RAW,&ts) calls - after the patch, the same call on the same machine has a latency of @ 20ns. Please consider applying something like this patch to a future Linux release. Thanks & Best Regards, Jason Vas Dias