From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751456AbeCNEUf (ORCPT ); Wed, 14 Mar 2018 00:20:35 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:54124 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750852AbeCNEUe (ORCPT ); Wed, 14 Mar 2018 00:20:34 -0400 X-Google-Smtp-Source: AG47ELtvqGpPw1CEPswhdBMPZfZl0sokgyB1OuqYNAi8ONo77XJTQryQBNpZzdBg59ptGToI5B3EGg== 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-rc5 (3)] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW Date: Wed, 14 Mar 2018 04:20:19 +0000 Message-Id: <1521001222-10712-1-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 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 @ 24ns 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/vdso/vdso.lds.S arch/x86/entry/vdso/vdsox32.lds.S arch/x86/entry/vdso/vdso32/vdso32.lds.S arch/x86/entry/vsyscall/vsyscall_gtod.c There are 3 patches in the series : Patch #1 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with rdtsc_ordered() Patch #2 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with a new rdtscp() function in msr.h Patch #3 makes the VDSO export TSC calibration data via a new function in the vDSO: unsigned int __vdso_linux_tsc_calibration ( struct linux_tsc_calibration *tsc_cal ) that user code can optionally call. Patches #2 & #3 should be considered "optional" . Patch #2 makes clock_gettime(CLOCK_MONOTONIC_RAW) calls have @ half the latency of clock_gettime(CLOCK_MONOTONIC) calls. I think something like Patch #3 is necessary to export TSC calibration data to user-space TSC readers. Best Regards, Jason Vas Dias