On 07/06/2016 09:59, Wanpeng Li wrote: > 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 underflow. > > Rik also pointed out to me: > > | I have seen stuff like that with live migration too, in the past > > The root cause of steal clock warp during hotplug is kvm_steal_time reset > to 0 after cpu hotplug comes back which should be preexiting guest value. > This patch fix it by don't reset kvm_steal_time during guest cpu hotplug. Improved commit message: Sometimes, after CPU hotplug you can observe a spike in stolen time (100%) followed by the CPU being marked as 100% idle when it's actually busy with a CPU hog task. The trace looks like the following: 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 sudden decrease of "steal" causes steal_jiffies to underflow. The root cause is kvm_steal_time being reset to 0 after hot-plugging back in a CPU. Instead, the preexisting value can be used, which is what the core scheduler code expects. John Stultz also reported a similar issue after guest S3. ------ Please also add Cc: John Stultz <john.stultz@xxxxxxxxxx> Thanks, Paolo -- 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
