On Tue, Aug 03, 2021 at 04:58:00PM +0800, Chao Gao wrote: > On Mon, Aug 02, 2021 at 10:02:57AM -0700, Paul E. McKenney wrote: > >On Mon, Aug 02, 2021 at 02:20:09PM +0800, Chao Gao wrote: > >> [snip] > >> >commit 48ebcfbfd877f5d9cddcc03c91352a8ca7b190af > >> >Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > >> >Date: Thu May 27 11:03:28 2021 -0700 > >> > > >> > clocksource: Forgive repeated long-latency watchdog clocksource reads > >> > > >> > Currently, the clocksource watchdog reacts to repeated long-latency > >> > clocksource reads by marking that clocksource unstable on the theory that > >> > these long-latency reads are a sign of a serious problem. And this theory > >> > does in fact have real-world support in the form of firmware issues [1]. > >> > > >> > However, it is also possible to trigger this using stress-ng on what > >> > the stress-ng man page terms "poorly designed hardware" [2]. And it > >> > is not necessarily a bad thing for the kernel to diagnose cases where > >> > high-stress workloads are being run on hardware that is not designed > >> > for this sort of use. > >> > > >> > Nevertheless, it is quite possible that real-world use will result in > >> > some situation requiring that high-stress workloads run on hardware > >> > not designed to accommodate them, and also requiring that the kernel > >> > refrain from marking clocksources unstable. > >> > > >> > Therefore, provide an out-of-tree patch that reacts to this situation > >> > by leaving the clocksource alone, but using the old 62.5-millisecond > >> > skew-detection threshold in response persistent long-latency reads. > >> > In addition, the offending clocksource is marked for re-initialization > >> > in this case, which both restarts that clocksource with a clean bill of > >> > health and avoids false-positive skew reports on later watchdog checks. > >> > >> Hi Paul, > >> > >> Sorry to dig out this old thread. > > > >Not a problem, especially given that this is still an experimental patch > >(marked with "EXP" in -rcu). So one remaining question is "what is this > >patch really supposed to do, if anything?". > > We are testing with TDX [1] and analyzing why kernel in a TD, or Trust Domain, > sometimes spots a large TSC skew. We have inspected tsc hardware/ucode/tdx > module to ensure no hardware issue, and also ported tsc_sync.c to a userspace > tool such that this tool can help to constantly check if tsc is synchronized > when some workload is running. Finally, we believe that the large TSC skew > spotted by TD kernel is a false positive. > > Your patches (those are merged) have improved clocksource watchdog a lot to > reduce false-positives. But due to the nature of TDX, switching between TD > and host takes more time. Then, the time window between two reads from > watchdog clocksource in cs_watchdog_read() increases, so does the > probability of the two reads being interrupted by whatever on host. Then, > sometimes, especially when there are heavy workloads in both host and TD, > the maximum number of retries in cs_watchdog_read() is exceeded and tsc is > marked unstable. > > Then we apply this out-of-tree patch, it helps to further reduce > false-positives. But TD kernel still observes TSC skew in some cases. After > a close look into kernel logs, we find patterns in those cases: an expected > re-initialization somehow doesn't happen. That's why we raise this issue > and ask for your advice. I am glad that the patch at least helps. ;-) > [1]: https://software.intel.com/content/www/us/en/develop/articles/intel-trust-domain-extensions.html > > >And here the clocksource failed the coarse-grained check and marked > >the clocksource as unstable. Perhaps because the previous read > >forced a coarse-grained check. Except that this should have forced > >a reinitialization. Ah, it looks like I need to suppress setting > >CLOCK_SOURCE_WATCHDOG if coarse-grained checks have been enabled. > >That could cause false-positive failure for the next check, after all. > > > >And perhaps make cs_watchdog_read() modify its print if there is > >a watchdog reset pending or if the current clocksource has the > >CLOCK_SOURCE_WATCHDOG flag cleared. > > > >Perhaps as shown in the additional patch below, to be folded into the > >original? > > Thanks. Will test with below patch applied. If this patch helps, but problems remain, another thing to try is to increase the clocksource.max_cswd_read_retries kernel boot parameter above its default value of 3. Maybe to 5 or 10? If this patch does not help, please let me know. In that case, there are probably more fixes required. Thanx, Paul > Thanks > Chao > > > > Thanx, Paul > > > >------------------------------------------------------------------------ > > > >diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > >index cfa992250c388..62da2485fd574 100644 > >--- a/kernel/time/clocksource.c > >+++ b/kernel/time/clocksource.c > >@@ -230,8 +230,13 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow) > > } > > } > > > >- pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, coarse-grained skew check followed by re-initialization\n", > >- smp_processor_id(), watchdog->name, wd_delay, nretries); > >+ if ((cs->flags & CLOCK_SOURCE_WATCHDOG) && !atomic_read(&watchdog_reset_pending)) { > >+ pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, coarse-grained skew check followed by re-initialization\n", > >+ smp_processor_id(), watchdog->name, wd_delay, nretries); > >+ } else { > >+ pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, awaiting re-initialization\n", > >+ smp_processor_id(), watchdog->name, wd_delay, nretries); > >+ } > > return true; > > } > > > >@@ -379,7 +384,8 @@ static void clocksource_watchdog(struct timer_list *unused) > > /* Clocksource initialized ? */ > > if (!(cs->flags & CLOCK_SOURCE_WATCHDOG) || > > atomic_read(&watchdog_reset_pending)) { > >- cs->flags |= CLOCK_SOURCE_WATCHDOG; > >+ if (!coarse) > >+ cs->flags |= CLOCK_SOURCE_WATCHDOG; > > cs->wd_last = wdnow; > > cs->cs_last = csnow; > > continue;
