Messages in this thread Patch in this message | | | Date | Fri, 23 Apr 2021 14:14:44 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression |
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On Fri, Apr 23, 2021 at 09:14:49PM +0200, Thomas Gleixner wrote: > On Thu, Apr 22 2021 at 07:24, Paul E. McKenney wrote: > > On Thu, Apr 22, 2021 at 03:41:26PM +0800, Feng Tang wrote: > > So what are our options? > > > > 1. Clear CLOCK_SOURCE_MUST_VERIFY from tsc-early. > > > > 2. #1, but add tsc-early into the watchdog list and set > > CLOCK_SOURCE_MUST_VERIFY once it is better calibrated. > > > > 3. Add a field to struct clocksource that, if non-zero, gives > > the maximum drift in nanoseconds per half second (AKA > > WATCHDOG_INTERVAL). If zero, the WATCHDOG_MAX_SKEW value > > is used. Set this to (say) 150,000ns for tsc-early. > > > > 4. As noted earlier, increase WATCHDOG_MAX_SKEW to 150 microseconds, > > which again is not a good approach given the real-world needs > > of real-world applications. > > > > 5. Your ideas here. > > #3 or add a flag to the clocksource which says 'frequency is guesswork' and > increase the threshold based on that. > > If that flag is still set max_drift is != 0 after 20 seconds yell.
I made it 60 seconds based on recent experience with large systems, but sounds good!
And the calls to clocksource_unregister(&clocksource_tsc_early) mean that it is not necessary to actually clear the .max_drift field, if I understand correctly.
It looks to me that init_tsc_clocksource() is invoked at device_initcall() time, and that it either immediately unregisters clocksource_tsc_early (known TSC frequency), or schedules a delayed work to make tsc_refine_calibration_work() do the calibration and the unregister.
Please see below for an untested patch.
Thanx, Paul
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diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 56289170753c..7192b8950322 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -1127,6 +1127,7 @@ static int tsc_cs_enable(struct clocksource *cs) static struct clocksource clocksource_tsc_early = { .name = "tsc-early", .rating = 299, + .max_drift = 5 * NSEC_PER_MSEC, .read = read_tsc, .mask = CLOCKSOURCE_MASK(64), .flags = CLOCK_SOURCE_IS_CONTINUOUS | diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index 83a3ebff7456..0c4418b381bf 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -93,6 +93,7 @@ struct clocksource { u32 shift; u64 max_idle_ns; u32 maxadj; + u32 max_drift; #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA struct arch_clocksource_data archdata; #endif diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index a1f90e2b1039..871837c46c62 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -126,6 +126,7 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating); */ #define WATCHDOG_INTERVAL (HZ >> 1) #define WATCHDOG_THRESHOLD (200 * NSEC_PER_USEC) +#define WATCHDOG_SYNC_FORGIVENESS (HZ * 60UL) /* * Maximum permissible delay between two readouts of the watchdog @@ -377,6 +378,7 @@ static void clocksource_watchdog(struct timer_list *unused) int next_cpu, reset_pending; int64_t wd_nsec, cs_nsec; struct clocksource *cs; + u32 md; spin_lock(&watchdog_lock); if (!watchdog_running) @@ -423,6 +425,12 @@ static void clocksource_watchdog(struct timer_list *unused) continue; /* Check the deviation from the watchdog clocksource. */ + if (!cs->max_drift) { + md = WATCHDOG_MAX_SKEW; + } else { + WARN_ON_ONCE(time_after(jiffies, WATCHDOG_SYNC_FORGIVENESS)); + md = cs->max_drift; + } if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) { pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n", smp_processor_id(), cs->name);
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