Re: [Qemu-devel] [PATCH] cpus: reset throttle_thread_scheduled after sleep

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Felipe Franciosi (felipe@nutanix.com) wrote:
> 
> > On 1 Jun 2017, at 15:36, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> > 
> > * Jason J. Herne (jjherne@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 <felipe@nutanix.com>
> >>> Signed-off-by: Malcolm Crossley <malcolm@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 <jjherne@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.

OK, that's reasonable.

> In summary we believe this patch is immediately required to prevent the lockups.

Yes, agreed.

> 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.

Dave

> 
> Thanks,
> Felipe
> 
> > 
> > Dave
> > 
> >> -- 
> >> -- Jason J. Herne (jjherne@linux.vnet.ibm.com)
> >> 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK