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

From: Daniel Lezcano
Date: Thu Nov 16 2017 - 04:45:35 EST

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" ?

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.

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