From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> I observed that sometimes st is 100% instantaneous, then idle is 100% even if there is a cpu hog on the guest cpu after the cpu hotplug comes back(N.B. this can not always be readily reproduced). I add trace to capture it as below: cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0 cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies = 1291 <idle>-0 [001] d.h1 167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000 <idle>-0 [001] d.h1 167.462664: account_process_tick: steal_jiffies = 18446744072437 The steal clock warp and then steal_jiffies overflow. Rik also pointed out to me: | I have seen stuff like that with live migration too, in the past This patch adds steal clock warp handling by a safe threshold to only apply steal times that are positive and smaller than one second (as long as nohz_full has the one second timer tick left), ignoring intervals that are negative or longer than a second, and using those to sync up the guest with the host. Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Radim <rkrcmar@xxxxxxxxxx> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> --- v1 -> v2: * update patch subject, description and comments * deal with the case where steal time suddenly increases by a ludicrous amount kernel/sched/cputime.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index f51c98c..751798a 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -262,17 +262,28 @@ static __always_inline unsigned long steal_account_process_tick(void) #ifdef CONFIG_PARAVIRT if (static_key_false(¶virt_steal_enabled)) { u64 steal; + s64 delta; unsigned long steal_jiffies; steal = paravirt_steal_clock(smp_processor_id()); - steal -= this_rq()->prev_steal_time; + delta = steal - this_rq()->prev_steal_time; + /* + * Ignore this steal time difference if the guest and the host got + * out of sync. This can happen due to events like live migration, + * or CPU hotplug. The upper threshold is set to one second to match + * the one second timer tick with nohz_full. + */ + if (unlikely(delta < 0 || delta > NSEC_PER_SEC)) { + this_rq()->prev_steal_time = steal; + return 0; + } /* * steal is in nsecs but our caller is expecting steal * time in jiffies. Lets cast the result to jiffies * granularity and account the rest on the next rounds. */ - steal_jiffies = nsecs_to_jiffies(steal); + steal_jiffies = nsecs_to_jiffies(delta); this_rq()->prev_steal_time += jiffies_to_nsecs(steal_jiffies); account_steal_time(jiffies_to_cputime(steal_jiffies)); -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html