https://bugzilla.kernel.org/show_bug.cgi?id=213029 Bug ID: 213029 Summary: [5.12 Regression] clock_gettime() a lot slower with Hyper-V clocksource driver Product: Drivers Version: 2.5 Kernel Version: 5.12 Hardware: x86-64 OS: Linux Tree: Mainline Status: NEW Severity: normal Priority: P1 Component: Platform_x86 Assignee: drivers_platform_x86@xxxxxxxxxxxxxxxxxxxx Reporter: mgamal@xxxxxxxxxx Regression: No Since kernel 5.12 the performance of clock_gettime() is a lot slower. There seems to be a regression concerning VDSO in the Hyper-V clocksource driver. The following code snippet runs a lot slower: =============================== # gcc gettime.c -o gettime # cat gettime.c #include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <time.h> int main(int argc, char *argv[]) { struct timespec start; unsigned long i; for (i=0; i<100000000UL; i++) { if( clock_gettime(CLOCK_REALTIME, &start) == -1 ) { perror( "clock gettime" ); exit( EXIT_FAILURE ); } } return 0; } =============================== # cat /sys/devices/system/clocksource/clocksource0/current_clocksource hyperv_clocksource_tsc_page 1) With kernel >= 5.12 # time ./gettime real 0m48.201s user 0m14.788s sys 0m31.531s 2) With kernel < 5.12 # time ./gettime real 0m3.308s user 0m3.214s sys 0m0.003s I bisected the kernel and the commit that introduces the regression seems to be this: commit e4ab4658f1cff14c82202132f7af2cb5c2741469 Author: Michael Kelley <mikelley@xxxxxxxxxxxxx> Date: Tue Mar 2 13:38:19 2021 -0800 clocksource/drivers/hyper-v: Handle vDSO differences inline While the driver for the Hyper-V Reference TSC and STIMERs is architecture neutral, vDSO is implemented for x86/x64, but not for ARM64. Current code calls into utility functions under arch/x86 (and coming, under arch/arm64) to handle the difference. Change this approach to handle the difference inline based on whether VDSO_CLOCK_MODE_HVCLOCK is present. The new approach removes code under arch/* since the difference is tied more to the specifics of the Linux implementation than to the architecture. No functional change. Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx> Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx> Acked-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> Link: https://lore.kernel.org/r/1614721102-2241-8-git-send-email-mikelley@xxxxxxxxxxxxx Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx> It does indeed not introduce any functional changes, but degradation in performance is quite noticeable. -- You may reply to this email to add a comment. You are receiving this mail because: You are watching the assignee of the bug.
