Date:   Mon, 20 Nov 2017 19:01:14 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Quan Xu <quan.xu0@...il.com>, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Quan Xu <quan.xu03@...il.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Kyle Huey <me@...ehuey.com>, Len Brown <len.brown@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: [PATCH RFC v3 3/6] sched/idle: Add a generic poll before enter
 real idle path

On 20/11/2017 08:05, Quan Xu wrote:

[ ... ]

>>>> 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.

The irq timings gives you an indication of the next interrupt deadline.

This information is a piece of the puzzle, you need to combine it with
the next timer expiration, and the next scheduling event. Then take the
earliest event in a timeline basis.

Using the trivial scheme above will work well with workload like videos
or mp3 but will fail as soon as the interrupts are not coming in a
regular basis and this is where the pattern recognition algorithm must act.

>> 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.

Sure, I will be glad if we can collaborate. I have some draft code but
before sharing it I would like we define what is the rate and what kind
of information we expect to infer from it. From my point of view it is a
value indicating the interrupt period per CPU, a short value indicates a
high number of interrupts on the CPU.

This value must decay with the time, the question here is what decay
function we apply to the rate from the last timestamp ?




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