| Message ID | 1441840051-20244-2-git-send-email-john.stultz@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Wed, Sep 09, 2015 at 04:07:31PM -0700, John Stultz wrote: > Recently an issue was reported that was difficult to detect except > by tweaking the adjtimex tick value, and noticing how quickly the > adjustment took to be made: > https://lkml.org/lkml/2015/9/1/488 > > Thus this patch introduces a new test which manipulates the adjtimex > tick value and validates the results are what we expect. > + if (llabs(eppm - ppm) > 10) { > + printf(" [FAILED]\n"); > + return -1; > + } > + printf(" [OK]\n"); > + return 0; This seems to work nicely with the tsc and hpet clocksources, but for some reason 10 ppm is not enough with the acpi_pm clocksource on both machines I tried this on. They both show -99988 ppm for the first test. When I modify the program to go through errors I get: Estimating tick (act: 9000 usec, -100000 ppm): 9001 usec, -99988 ppm [FAILED] Estimating tick (act: 9250 usec, -75000 ppm): 9251 usec, -74991 ppm [OK] Estimating tick (act: 9500 usec, -50000 ppm): 9501 usec, -49994 ppm [OK] Estimating tick (act: 9750 usec, -25000 ppm): 9751 usec, -24997 ppm [OK] Estimating tick (act: 10000 usec, 0 ppm): 10000 usec, 0 ppm [OK] Estimating tick (act: 10250 usec, 25000 ppm): 10249 usec, 24996 ppm [OK] Estimating tick (act: 10500 usec, 50000 ppm): 10499 usec, 49993 ppm [OK] Estimating tick (act: 10750 usec, 75000 ppm): 10749 usec, 74990 ppm [OK] The precision of the clock is better than microsecond, so that wouldn't explain a 12 ppm error over the 15 second interval. I guess it's due to a larger xtime_remainder, which basically is a hidden frequency offset added (and not multiplied) to the NTP frequency offset. Would that explain it?
On Thu, Sep 10, 2015 at 5:02 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: > On Wed, Sep 09, 2015 at 04:07:31PM -0700, John Stultz wrote: >> Recently an issue was reported that was difficult to detect except >> by tweaking the adjtimex tick value, and noticing how quickly the >> adjustment took to be made: >> https://lkml.org/lkml/2015/9/1/488 >> >> Thus this patch introduces a new test which manipulates the adjtimex >> tick value and validates the results are what we expect. > >> + if (llabs(eppm - ppm) > 10) { >> + printf(" [FAILED]\n"); >> + return -1; >> + } >> + printf(" [OK]\n"); >> + return 0; > > This seems to work nicely with the tsc and hpet clocksources, but for > some reason 10 ppm is not enough with the acpi_pm clocksource on both > machines I tried this on. They both show -99988 ppm for the first > test. When I modify the program to go through errors I get: > > Estimating tick (act: 9000 usec, -100000 ppm): 9001 usec, -99988 ppm [FAILED] > Estimating tick (act: 9250 usec, -75000 ppm): 9251 usec, -74991 ppm [OK] > Estimating tick (act: 9500 usec, -50000 ppm): 9501 usec, -49994 ppm [OK] > Estimating tick (act: 9750 usec, -25000 ppm): 9751 usec, -24997 ppm [OK] > Estimating tick (act: 10000 usec, 0 ppm): 10000 usec, 0 ppm [OK] > Estimating tick (act: 10250 usec, 25000 ppm): 10249 usec, 24996 ppm [OK] > Estimating tick (act: 10500 usec, 50000 ppm): 10499 usec, 49993 ppm [OK] > Estimating tick (act: 10750 usec, 75000 ppm): 10749 usec, 74990 ppm [OK] > > The precision of the clock is better than microsecond, so that > wouldn't explain a 12 ppm error over the 15 second interval. I guess > it's due to a larger xtime_remainder, which basically is a hidden > frequency offset added (and not multiplied) to the NTP frequency > offset. Would that explain it? I think its due to the ntp_error being large enough prior (or during the freq transition) that we're still applying a single unit freq adjustment for that error. But I'm guessing on the acpi_pm clocksource the shift is low enough that a single unit adjustment is coarse enough to affect the ppm, since I see the same consistently measured ppm result if I both increase the settling time measurement sleep times. If I left it for a long long time, the single unit correction would likely null the error out and we'd get the desired result, but I don't think the test has time for that. The short term answer is to likely up the acceptable range for passing the test. Long term, we can look at further improving the error accumulation. I'm thinking your earlier approach of doing the more expensive division instead of the approximation over a series of ticks might reduce the error generated during that transition. So that might be one approach. Pondering a bit on this, I'm thinking while its ideally nice to keep the ntp_error true to the difference between where the system time is and where its been told to be, I'm not if that full history makes total sense. As if ntpd has specified a different frequency, it may not make since to try to correct the accumulated error from the past. Since at that point, if ntpd has looked at where we are and is specifying a new freq, in some ways its accounting for the current uncorrected error. So we might just consider clearing the ntp_error after the approximation is finished. Though I probably need to think on this approach a bit more. Your thoughts? thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Thu, Sep 10, 2015 at 10:42 AM, John Stultz <john.stultz@linaro.org> wrote: > On Thu, Sep 10, 2015 at 5:02 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: >> On Wed, Sep 09, 2015 at 04:07:31PM -0700, John Stultz wrote: >>> Recently an issue was reported that was difficult to detect except >>> by tweaking the adjtimex tick value, and noticing how quickly the >>> adjustment took to be made: >>> https://lkml.org/lkml/2015/9/1/488 >>> >>> Thus this patch introduces a new test which manipulates the adjtimex >>> tick value and validates the results are what we expect. >> >>> + if (llabs(eppm - ppm) > 10) { >>> + printf(" [FAILED]\n"); >>> + return -1; >>> + } >>> + printf(" [OK]\n"); >>> + return 0; >> >> This seems to work nicely with the tsc and hpet clocksources, but for >> some reason 10 ppm is not enough with the acpi_pm clocksource on both >> machines I tried this on. They both show -99988 ppm for the first >> test. When I modify the program to go through errors I get: >> >> Estimating tick (act: 9000 usec, -100000 ppm): 9001 usec, -99988 ppm [FAILED] >> Estimating tick (act: 9250 usec, -75000 ppm): 9251 usec, -74991 ppm [OK] >> Estimating tick (act: 9500 usec, -50000 ppm): 9501 usec, -49994 ppm [OK] >> Estimating tick (act: 9750 usec, -25000 ppm): 9751 usec, -24997 ppm [OK] >> Estimating tick (act: 10000 usec, 0 ppm): 10000 usec, 0 ppm [OK] >> Estimating tick (act: 10250 usec, 25000 ppm): 10249 usec, 24996 ppm [OK] >> Estimating tick (act: 10500 usec, 50000 ppm): 10499 usec, 49993 ppm [OK] >> Estimating tick (act: 10750 usec, 75000 ppm): 10749 usec, 74990 ppm [OK] >> >> The precision of the clock is better than microsecond, so that >> wouldn't explain a 12 ppm error over the 15 second interval. I guess >> it's due to a larger xtime_remainder, which basically is a hidden >> frequency offset added (and not multiplied) to the NTP frequency >> offset. Would that explain it? > > I think its due to the ntp_error being large enough prior (or during > the freq transition) that we're still applying a single unit freq > adjustment for that error. But I'm guessing on the acpi_pm clocksource > the shift is low enough that a single unit adjustment is coarse enough > to affect the ppm, since I see the same consistently measured ppm > result if I both increase the settling time measurement sleep times. > If I left it for a long long time, the single unit correction would > likely null the error out and we'd get the desired result, but I don't > think the test has time for that. > > The short term answer is to likely up the acceptable range for passing > the test. So bumping the fail level to > 100ppm avoids false positives due to long-term error correction with coarse clocksources, but still is tight enough to catch the dampened approximation issue caused by the abs(s64) problem. Any objection to moving to that? It is still a 0.01% error bound. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Thu, Sep 10, 2015 at 11:14:25AM -0700, John Stultz wrote: > On Thu, Sep 10, 2015 at 10:42 AM, John Stultz <john.stultz@linaro.org> wrote: > > On Thu, Sep 10, 2015 at 5:02 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: > >> The precision of the clock is better than microsecond, so that > >> wouldn't explain a 12 ppm error over the 15 second interval. I guess > >> it's due to a larger xtime_remainder, which basically is a hidden > >> frequency offset added (and not multiplied) to the NTP frequency > >> offset. Would that explain it? > > > > I think its due to the ntp_error being large enough prior (or during > > the freq transition) that we're still applying a single unit freq > > adjustment for that error. But I'm guessing on the acpi_pm clocksource > > the shift is low enough that a single unit adjustment is coarse enough > > to affect the ppm, since I see the same consistently measured ppm > > result if I both increase the settling time measurement sleep times. > > If I left it for a long long time, the single unit correction would > > likely null the error out and we'd get the desired result, but I don't > > think the test has time for that. I ran few tests and it doesn't seem to be a problem with large ntp_error or an extremely slow adjustment of the multiplier for the new frequency. I think it really is the xtime_remainder correction. It is a fixed offset added to the ntp error on each tick to compensate for the cycle_interval rounding error. With the acpi_pm clocksource and 1000Hz update rate xtime_remainder is -127 ns, which effectively speeds up the clock by 127 ppm. When NTP slows the clock down by 10%, the correction is not decreased by 10% and we can observe the clock is running faster by 12.7 ppm than expected. Is there a cheap way to calculate this? xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH > So bumping the fail level to > 100ppm avoids false positives due to > long-term error correction with coarse clocksources, but still is > tight enough to catch the dampened approximation issue caused by the > abs(s64) problem. > > Any objection to moving to that? It is still a 0.01% error bound. No objection from me as long as we understand where that error is coming from.
On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: > On Thu, Sep 10, 2015 at 11:14:25AM -0700, John Stultz wrote: >> On Thu, Sep 10, 2015 at 10:42 AM, John Stultz <john.stultz@linaro.org> wrote: >> > On Thu, Sep 10, 2015 at 5:02 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: >> >> The precision of the clock is better than microsecond, so that >> >> wouldn't explain a 12 ppm error over the 15 second interval. I guess >> >> it's due to a larger xtime_remainder, which basically is a hidden >> >> frequency offset added (and not multiplied) to the NTP frequency >> >> offset. Would that explain it? >> > >> > I think its due to the ntp_error being large enough prior (or during >> > the freq transition) that we're still applying a single unit freq >> > adjustment for that error. But I'm guessing on the acpi_pm clocksource >> > the shift is low enough that a single unit adjustment is coarse enough >> > to affect the ppm, since I see the same consistently measured ppm >> > result if I both increase the settling time measurement sleep times. >> > If I left it for a long long time, the single unit correction would >> > likely null the error out and we'd get the desired result, but I don't >> > think the test has time for that. > > I ran few tests and it doesn't seem to be a problem with large > ntp_error or an extremely slow adjustment of the multiplier for the > new frequency. > > I think it really is the xtime_remainder correction. It is a fixed > offset added to the ntp error on each tick to compensate for the > cycle_interval rounding error. With the acpi_pm clocksource and 1000Hz > update rate xtime_remainder is -127 ns, which effectively speeds up > the clock by 127 ppm. When NTP slows the clock down by 10%, the > correction is not decreased by 10% and we can observe the clock is > running faster by 12.7 ppm than expected. Sorry for taking so long to get back to you here. Had a conference (and related prep) that pulled me away. So yea.. I've spend some more time looking at this, and your argument above looks pretty convincing and my theory didn't prove out (clearing out the ntp_error value on frequency changes doesn't avoid the issue). > Is there a cheap way to calculate this? > xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH Hrm.. So xtime_remainder = (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift) - (tk->cycle_interval * tk->tkr_mono.clock->mult) for simplificiation: And we want to scale it as you pointed out above (though slightly fixed here) by: (tk->ntp_tick >> tk->ntp_error_shift) / (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift) So this comes out to: (tk->ntp_tick ) - (tk->ntp_tick ) * (tk->cycle_interval * tk->tkr_mono.clock->mult) / (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift) tk->ntp_error_shift would: xtime_remainder = (tk->ntp_tick >> ntp_error_shift) - tk->xtime_interval After we've adjusted xtime_interval give us the equivalent? >> So bumping the fail level to > 100ppm avoids false positives due to >> long-term error correction with coarse clocksources, but still is >> tight enough to catch the dampened approximation issue caused by the >> abs(s64) problem. >> >> Any objection to moving to that? It is still a 0.01% error bound. > > No objection from me as long as we understand where that error is > coming from. Yea. I think we agree. I did find one bug with the test (we can't clear an existing offset if STA_PLL isn't set), so I'll be resubmitting it here in a bit. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Fri, Oct 2, 2015 at 1:25 PM, John Stultz <john.stultz@linaro.org> wrote: > On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: >> Is there a cheap way to calculate this? >> xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH > > > > Hrm.. So > xtime_remainder = (NTP_INTERVAL_LENGTH << > tk->tkr_mono.clock->shift) - (tk->cycle_interval * > tk->tkr_mono.clock->mult) > > for simplificiation: > > And we want to scale it as you pointed out above (though slightly > fixed here) by: > (tk->ntp_tick >> tk->ntp_error_shift) / (NTP_INTERVAL_LENGTH > << tk->tkr_mono.clock->shift) > > > So this comes out to: > > > (tk->ntp_tick ) - (tk->ntp_tick ) * (tk->cycle_interval * > tk->tkr_mono.clock->mult) / (NTP_INTERVAL_LENGTH << > tk->tkr_mono.clock->shift) > tk->ntp_error_shift > > > > would: > > xtime_remainder = (tk->ntp_tick >> ntp_error_shift) - tk->xtime_interval > > After we've adjusted xtime_interval give us the equivalent? Bah.. Ignore the above here. I was still working out a calculation and accidentally hit send. -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
On Mon, Sep 14, 2015 at 7:48 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote: > Is there a cheap way to calculate this? > xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH So as I was trying to figure out before I prematurely hit send... I was thinking... xtime_remainder = (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift) - (tk->cycle_interval * tk->tkr_mono.clock->mult) or for simplification: xtime_remainder = NTP_INTERVAL_SHIFTED - ORIG_MULT_INTERVAL_SHIFTED And we want to scale it as you pointed out above (though slightly fixed here) by: (tk->ntp_tick >> tk->ntp_error_shift) / (NTP_INTERVAL_LENGTH << tk->tkr_mono.clock->shift) Which we'll simplify a touch as: (tk->ntp_tick >> tk->ntp_error_shift) / NTP_INTERVAL_SHIFTED So multiplying these comes out to: (tk->ntp_tick >> tk->ntp_error_shift) - ((tk->ntp_tick >> tk->ntp_error_shift) * ORIG_MULT_INTERVAL_SHIFTED / NTP_INTERVAL_SHIFTED) Which simplifies a bit to: (tk->ntp_tick - (tk->ntp_tick * ORIG_MULT_INTERVAL_SHIFTED / NTP_INTERVAL_SHIFTED)) >> tk->ntp_error_shift (tk->ntp_tick * ( 1 - ORIG_MULT_INTERVAL_SHIFTED / NTP_INTERVAL_SHIFTED)) >> tk->ntp_error_shift; So this looks like it would be cheap to calculate, but since we're doing integer math, the problem is that ORIG_MULT_INTERVAL_SHIFTED / NTP_INTERVAL_SHIFTED isn't an integer value, so if we want any precision we'd still have the costly tk->ntp_tick/NTP_INTERVAL_SHIFTED to do each time. So.. that's a long winded way to say I can't think of a cheap way.... :P thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/tools/testing/selftests/timers/Makefile b/tools/testing/selftests/timers/Makefile index 89a3f44..4a1be1b 100644 --- a/tools/testing/selftests/timers/Makefile +++ b/tools/testing/selftests/timers/Makefile @@ -8,7 +8,7 @@ LDFLAGS += -lrt -lpthread TEST_PROGS = posix_timers nanosleep nsleep-lat set-timer-lat mqueue-lat \ inconsistency-check raw_skew threadtest rtctest -TEST_PROGS_EXTENDED = alarmtimer-suspend valid-adjtimex change_skew \ +TEST_PROGS_EXTENDED = alarmtimer-suspend valid-adjtimex adjtick change_skew \ skew_consistency clocksource-switch leap-a-day \ leapcrash set-tai set-2038 @@ -24,6 +24,7 @@ include ../lib.mk run_destructive_tests: run_tests ./alarmtimer-suspend ./valid-adjtimex + ./adjtick ./change_skew ./skew_consistency ./clocksource-switch diff --git a/tools/testing/selftests/timers/adjtick.c b/tools/testing/selftests/timers/adjtick.c new file mode 100644 index 0000000..c6efcec --- /dev/null +++ b/tools/testing/selftests/timers/adjtick.c @@ -0,0 +1,199 @@ +/* adjtimex() tick adjustment test + * by: John Stultz <john.stultz@linaro.org> + * (C) Copyright Linaro Limited 2015 + * Licensed under the GPLv2 + * + * To build: + * $ gcc adjtick.c -o adjtick -lrt + * + * This program is free software: you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation, either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + */ + +#include <stdio.h> +#include <unistd.h> +#include <stdlib.h> +#include <sys/time.h> +#include <sys/timex.h> +#include <time.h> +#ifdef KTEST +#include "../kselftest.h" +#else +static inline int ksft_exit_pass(void) +{ + exit(0); +} +static inline int ksft_exit_fail(void) +{ + exit(1); +} +#endif + + +#define CLOCK_MONOTONIC_RAW 4 +#define NSEC_PER_SEC 1000000000LL +#define USEC_PER_SEC 1000000 +#define MILLION 1000000 + +long systick; + +long long llabs(long long val) +{ + if (val < 0) + val = -val; + return val; +} + +unsigned long long ts_to_nsec(struct timespec ts) +{ + return ts.tv_sec * NSEC_PER_SEC + ts.tv_nsec; +} + +struct timespec nsec_to_ts(long long ns) +{ + struct timespec ts; + + ts.tv_sec = ns/NSEC_PER_SEC; + ts.tv_nsec = ns%NSEC_PER_SEC; + return ts; +} + +long long diff_timespec(struct timespec start, struct timespec end) +{ + long long start_ns, end_ns; + + start_ns = ts_to_nsec(start); + end_ns = ts_to_nsec(end); + return end_ns - start_ns; +} + +void get_monotonic_and_raw(struct timespec *mon, struct timespec *raw) +{ + struct timespec start, mid, end; + long long diff = 0, tmp; + int i; + + clock_gettime(CLOCK_MONOTONIC, mon); + clock_gettime(CLOCK_MONOTONIC_RAW, raw); + + /* Try to get a more tightly bound pairing */ + for (i = 0; i < 3; i++) { + long long newdiff; + + clock_gettime(CLOCK_MONOTONIC, &start); + clock_gettime(CLOCK_MONOTONIC_RAW, &mid); + clock_gettime(CLOCK_MONOTONIC, &end); + + newdiff = diff_timespec(start, end); + if (diff == 0 || newdiff < diff) { + diff = newdiff; + *raw = mid; + tmp = (ts_to_nsec(start) + ts_to_nsec(end))/2; + *mon = nsec_to_ts(tmp); + } + } +} + +long long get_ppm_drift(void) +{ + struct timespec mon_start, raw_start, mon_end, raw_end; + long long delta1, delta2, eppm; + + get_monotonic_and_raw(&mon_start, &raw_start); + + sleep(15); + + get_monotonic_and_raw(&mon_end, &raw_end); + + delta1 = diff_timespec(mon_start, mon_end); + delta2 = diff_timespec(raw_start, raw_end); + + eppm = (delta1*MILLION)/delta2 - MILLION; + return eppm; +} + +int check_tick_adj(long tickval) +{ + long long eppm, ppm; + struct timex tx1; + + tx1.modes = ADJ_TICK; + tx1.modes |= ADJ_OFFSET; + tx1.modes |= ADJ_FREQUENCY; + tx1.offset = 0; + tx1.freq = 0; + tx1.tick = tickval; + adjtimex(&tx1); + sleep(1); + + ppm = ((long long)tickval * MILLION)/systick - MILLION; + printf("Estimating tick (act: %ld usec, %lld ppm): ", tickval, ppm); + + eppm = get_ppm_drift(); + printf("%lld usec, %lld ppm", systick + (systick * eppm / MILLION), eppm); + + tx1.modes = 0; + adjtimex(&tx1); + if (tx1.offset || tx1.freq || tx1.tick != tickval) { + printf("WARNING: Unexpected adjtimex return values, make sure ntpd is not running. "); + return -1; + } + + if (llabs(eppm - ppm) > 10) { + printf(" [FAILED]\n"); + return -1; + } + printf(" [OK]\n"); + return 0; +} + +int main(int argv, char **argc) +{ + struct timespec raw; + long tick, max, interval, err; + struct timex tx1; + + err = 0; + setbuf(stdout, NULL); + + if (clock_gettime(CLOCK_MONOTONIC_RAW, &raw)) { + printf("ERR: NO CLOCK_MONOTONIC_RAW\n"); + return -1; + } + + + systick = sysconf(_SC_CLK_TCK); + systick = USEC_PER_SEC/sysconf(_SC_CLK_TCK); + printf("systick: %ld\n", systick); + + max = systick/10; /* +/- 10% */ + interval = max/4; /* in 4 steps each side */ + + for (tick = (systick - max); tick < (systick + max); + tick += interval) { + if (check_tick_adj(tick)) { + err = 1; + break; + } + } + + /* Reset things to zero */ + tx1.modes = ADJ_TICK; + tx1.modes |= ADJ_OFFSET; + tx1.modes |= ADJ_FREQUENCY; + tx1.offset = 0; + tx1.freq = 0; + tx1.tick = systick; + adjtimex(&tx1); + + if (err) + return ksft_exit_fail(); + return ksft_exit_pass(); +}
Recently an issue was reported that was difficult to detect except by tweaking the adjtimex tick value, and noticing how quickly the adjustment took to be made: https://lkml.org/lkml/2015/9/1/488 Thus this patch introduces a new test which manipulates the adjtimex tick value and validates the results are what we expect. Cc: Nuno Gonçalves <nunojpg@gmail.com> Cc: Miroslav Lichvar <mlichvar@redhat.com> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Shuah Khan <shuahkh@osg.samsung.com> Signed-off-by: John Stultz <john.stultz@linaro.org> --- v2: Use sysconf(_SC_CLK_TCK) to properly get USER_HZ value. tools/testing/selftests/timers/Makefile | 3 +- tools/testing/selftests/timers/adjtick.c | 199 +++++++++++++++++++++++++++++++ 2 files changed, 201 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/timers/adjtick.c