Messages in this thread | | | From | jason.vas.dias@gmail ... | Subject | [PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall | Date | Wed, 21 Mar 2018 16:32:04 +0000 |
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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-rc6 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.
This patch is being resent because it has slight improvements to vclock_gettime static function attributes wrt. the previous version.
It also supersedes all previous patches with subject matching '.*VDSO should handle.*clock_gettime.*MONOTONIC_RAW' that I have sent previously - sorry for the resends.
Please apply this patch so we stop getting emails from intel build bot trying to build previous version, with subject : '[PATCH v4.16-rc5 1/2] x86/vdso: VDSO should handle \ clock_gettime(CLOCK_MONOTONIC_RAW) without syscall' , which only fails to build because its patch 2/2 , which removed -DRETPOLINE from the VDSO build, and is now the subject of https://bugzilla.kernel.org/show_bug.cgi?id=199129, raised by H.J. Liu, was not applied first - Sorry!
Thanks & Best Regards, Jason Vas Dias
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