Re: [Xen-devel] [PATCH RFC v3 3/6] sched/idle: Add a generic poll before enter real idle path

[Quan Xu <quan.xu0@xxxxxxxxx>]

On 2017-11-16 17:45, Daniel Lezcano wrote:
> On 16/11/2017 10:12, Quan Xu wrote:
> > On 2017-11-16 06:03, Thomas Gleixner wrote:
> > > On Wed, 15 Nov 2017, Peter Zijlstra wrote:
> > >
> > > > On Mon, Nov 13, 2017 at 06:06:02PM +0800, Quan Xu wrote:
> > > > > From: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
> > > > >
> > > > > Implement a generic idle poll which resembles the functionality
> > > > > found in arch/. Provide weak arch_cpu_idle_poll function which
> > > > > can be overridden by the architecture code if needed.
> > > > No, we want less of those magic hooks, not more.
> > > >
> > > > > Interrupts arrive which may not cause a reschedule in idle loops.
> > > > > In KVM guest, this costs several VM-exit/VM-entry cycles, VM-entry
> > > > > for interrupts and VM-exit immediately. Also this becomes more
> > > > > expensive than bare metal. Add a generic idle poll before enter
> > > > > real idle path. When a reschedule event is pending, we can bypass
> > > > > the real idle path.
> > > > Why not do a HV specific idle driver?
> > > If I understand the problem correctly then he wants to avoid the heavy
> > > lifting in tick_nohz_idle_enter() in the first place, but there is
> > > already
> > > an interesting quirk there which makes it exit early.  See commit
> > > 3c5d92a0cfb5 ("nohz: Introduce arch_needs_cpu"). The reason for this
> > > commit
> > > looks similar. But lets not proliferate that. I'd rather see that go
> > > away.
> > agreed.
> >
> > Even we can get more benifit than commit 3c5d92a0cfb5 ("nohz: Introduce
> > arch_needs_cpu")
> > in kvm guest. I won't proliferate that..
> >
> > > But the irq_timings stuff is heading into the same direction, with a more
> > > complex prediction logic which should tell you pretty good how long that
> > > idle period is going to be and in case of an interrupt heavy workload
> > > this
> > > would skip the extra work of stopping and restarting the tick and
> > > provide a
> > > very good input into a polling decision.
> > interesting. I have tested with IRQ_TIMINGS related code, which seems
> > not working so far.
> I don't know how you tested it, can you elaborate what you meant by
> "seems not working so far" ?
Daniel, I tried to enable IRQ_TIMINGS* manually. used 
irq_timings_next_event()
to return estimation of the earliest interrupt. However I got a constant.

> There are still some work to do to be more efficient. The prediction
> based on the irq timings is all right if the interrupts have a simple
> periodicity. But as soon as there is a pattern, the current code can't
> handle it properly and does bad predictions.
>
> I'm working on a self-learning pattern detection which is too heavy for
> the kernel, and with it we should be able to detect properly the
> patterns and re-ajust the period if it changes. I'm in the process of
> making it suitable for kernel code (both math and perf).
>
> One improvement which can be done right now and which can help you is
> the interrupts rate on the CPU. It is possible to compute it and that
> will give an accurate information for the polling decision.
As tglx said, talk to each other / work together to make it usable for 
all use cases.
could you share how to enable it to get the interrupts rate on the CPU? 
I can try it
in cloud scenario. of course, I'd like to work with you to improve it.

Quan
Alibaba Cloud


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