Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers

[Yang Zhang]

On 2016/5/25 7:37, David Matlack wrote:
> On Tue, May 24, 2016 at 4:11 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> > 2016-05-25 6:38 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>:
> > > On Tue, May 24, 2016 at 12:57 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> > > > From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > > >
> > > > If an emulated lapic timer will fire soon(in the scope of 10us the
> > > > base of dynamic halt-polling, lower-end of message passing workload
> > > > latency TCP_RR's poll time < 10us) we can treat it as a short halt,
> > > > and poll to wait it fire, the fire callback apic_timer_fn() will set
> > > > KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
> > > > This can avoid context switch overhead and the latency which we wake
> > > > up vCPU.
> > > >
> > > > This feature is slightly different from current advance expiration
> > > > way. Advance expiration rely on the vCPU is running(do polling before
> > > > vmentry). But in some cases, the timer interrupt may be blocked by
> > > > other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to
> > > > run immediately. So even advance the timer early, vCPU may still see
> > > > the latency. But polling is different, it ensures the vCPU to aware
> > > > the timer expiration before schedule out.
> > > >
> > > > echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests.
> > > >
> > > > Context switching - times in microseconds - smaller is better
> > > > -------------------------------------------------------------------------
> > > > Host                 OS  2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
> > > >                          ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
> > > > --------- ------------- ------ ------ ------ ------ ------ ------- -------
> > > > kernel     Linux 4.6.0+ 7.9800   11.0   10.8   14.6 9.4300    13.0    10.2 vanilla
> > > > kernel     Linux 4.6.0+   15.3   13.6   10.7   12.5 9.0000    12.8 7.38000 poll
> > >
> > > These results aren't very compelling. Sometimes polling is faster,
> > > sometimes vanilla is faster, sometimes they are about the same.
> >
> > More processes and bigger cache footprints can get benefit from the
> > result since I open the hrtimer for the precision preemption.
>
> The VCPU is halted (idle), so the timer interrupt is not preempting
> anything. Also I would not expect any preemption in a context
> switching benchmark, the threads should be handing off execution to
> one another.
>
> I'm confused why timers would play any role in the performance of this
> benchmark. Any idea why there's a speedup in the 8p/16K and 16p/64K
> runs?
>
> > Actually
> > I try to emulate Yang's workload, https://lkml.org/lkml/2016/5/22/162.
> > And his real workload can get more benefit as he mentioned,
> > https://lkml.org/lkml/2016/5/19/667.
> >
> > > I imagine there are hyper sensitive workloads which cannot tolerate a
> > > long tail in timer latency (e.g. realtime workloads). I would expect a
> > > patch like this to provide a "smoothing effect", reducing that tail.
> > > But for cloud/server workloads, I would not expect any sensitivity to
> > > jitter in timer latency (especially while the VCPU is halted).
> >
> > Yang's is real cloud workload.
>
> I have 2 issues with optimizing for Yang's workload. Yang, please
> correct me if I am mis-characterizing it.
> 1. The delay in timer interrupts is caused by something disabling the
> interrupts on the CPU for more than a millisecond. It seems that is
> the real issue. I'm wary of using polling as a workaround.

Yes, this is the most likely case.

> 2. The delay is caused by a separate task. Halt-polling would not help
> in that scenario, it would yield the CPU to that task.

In some cases, the separate task is migrated from other CPU after CPU 
enter idle state. So Halt-polling may still help. And the delay is 
caused by two context switches(VCPU schedule out and migrate VCPU to 
another idle CPU).

> > > Note that while halt-polling happens when the CPU is idle, it's still
> > > not free. It constricts the scheduler's cpu load balancer, because the
> > > CPU appears to be busy. In KVM's default configuration, I'd prefer to
> > > only add more polling when the gain is clear. If there are guest
> > > workloads that want this patch, I'd suggest polling for timers be
> > > default-off. At minimum, there should be a module parameter to control
> > > it (like Christian Borntraeger suggested).
> >
> > Yeah, I will add the module parameter in order to enable/disable.
> >
> > Regards,
> > Wanpeng Li


--
best regards
yang