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Subject[PATCH v4.16-rc5 (3)] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW
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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

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