> On 1 Jun 2017, at 15:36, Dr. David Alan Gilbert <dgilb...@redhat.com> wrote: > > * Jason J. Herne (jjhe...@linux.vnet.ibm.com) wrote: >> On 05/19/2017 05:29 PM, Felipe Franciosi wrote: >>> Currently, the throttle_thread_scheduled flag is reset back to 0 before >>> sleeping (as part of the throttling logic). Given that throttle_timer >>> (well, any timer) may tick with a slight delay, it so happens that under >>> heavy throttling (ie. close or on CPU_THROTTLE_PCT_MAX) the tick may >>> schedule a further cpu_throttle_thread() work item after the flag reset, >>> but before the previous sleep completed. This results on the vCPU thread >>> sleeping continuously for potentially several seconds in a row. >>> >>> The chances of that happening can be drastically minimised by resetting >>> the flag after the sleep. >>> >>> Signed-off-by: Felipe Franciosi <fel...@nutanix.com> >>> Signed-off-by: Malcolm Crossley <malc...@nutanix.com> >>> --- >>> cpus.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/cpus.c b/cpus.c >>> index 516e5cb..f42eebd 100644 >>> --- a/cpus.c >>> +++ b/cpus.c >>> @@ -677,9 +677,9 @@ static void cpu_throttle_thread(CPUState *cpu, >>> run_on_cpu_data opaque) >>> sleeptime_ns = (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS); >>> >>> qemu_mutex_unlock_iothread(); >>> - atomic_set(&cpu->throttle_thread_scheduled, 0); >>> g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */ >>> qemu_mutex_lock_iothread(); >>> + atomic_set(&cpu->throttle_thread_scheduled, 0); >>> } >>> >>> static void cpu_throttle_timer_tick(void *opaque) >>> >> >> This seems to make sense to me. >> >> Acked-by: Jason J. Herne <jjhe...@linux.vnet.ibm.com> >> >> I'm CC'ing Juan, Amit and David as they are all active in the migration area >> and may have >> opinions on this. Juan and David were also reviewers for the original >> series. > > The description is interesting and sounds reasonable; it'll be > interesting to see what difference it makes to the autoconverge > behaviour for those workloads that need this level of throttle.
To get some hard data, we wrote a little application that: 1) spawns multiple threads (one per vCPU) 2) each thread mmap()s+mlock()s a certain workset (eg. 30GB/#threads for a 32GB VM) 3) each thread writes a word to the beginning of every page in a tight loop 4) the parent thread periodically reports the number of dirtied pages Even on a dedicated 10G link, that is pretty much guaranteed to require 99% throttle to converge. Before the patch, Qemu migrates the VM (depicted above) fairly quickly (~40s) after reaching 99% throttle. The application reported a few seconds at a time with lockups which we initially thought was just that thread not running between Qemu-induced vCPU sleeps (and later attributed it to the reported bug). Then we used a 1G link. This time, the migration had to run for a lot longer even at 99%. That made the bug more likely to happen and we observed soft lockups (reported by the guest's kernel on the console) of 70+ seconds. Using the patch, and back on a 10G link, the migration completes after a few more iterations than before (took just under 2mins after reaching 99%). If you want further validation of the bug, instrumenting cpus-common.c:process_queued_cpu_work() could be done to show that cpu_throttle_thread() is running back-to-back under these cases. In summary we believe this patch is immediately required to prevent the lockups. A more elaborate throttling solution should be considered as future work. Perhaps a per-vCPU timer which throttles more precisely or a new convergence design altogether. Thanks, Felipe > > Dave > >> -- >> -- Jason J. Herne (jjhe...@linux.vnet.ibm.com) >> > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
