Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <ak@linux.intel.com>, Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Christoph Lameter <cl@linux.com>, Aubrey Li <aubrey.li@intel.com>,
        len.brown@intel.com, rjw@rjwysocki.net, tim.c.chen@linux.intel.com,
        arjan@linux.intel.com, paulmck@linux.vnet.ibm.com,
        yang.zhang.wz@gmail.com, x86@kernel.org, linux-kernel@vger.kernel.org,
        daniel.lezcano@linaro.org
References: <20170713182820.sn3fjitnd3mca27p@hirez.programming.kicks-ass.net>
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 <348019f4-85ae-ba91-3fce-9886533e8d22@linux.intel.com>
 <alpine.DEB.2.20.1707190928510.2286@nanos>
From: "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <0c346d95-baae-715a-bff5-4738b924ccff@linux.intel.com>
Date: Thu, 20 Jul 2017 09:56:59 +0800
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On 2017/7/19 15:55, Thomas Gleixner wrote:
> On Wed, 19 Jul 2017, Li, Aubrey wrote:
>> On 2017/7/18 15:19, Thomas Gleixner wrote:
>>> You can very well avoid it by taking the irq timings or whatever other
>>> information into account for the NOHZ decision.
>>>
>> If I read the source correctly, irq timing statistics computation happens at
>> idle time. Sadly, this is what Andi Kleen worried about. People keep putting
>> more and more slow stuff in idle path, not realizing it could be a critical path.
> 
> Can you please stop this right here? We all realize that there is an issue,
> we are not as stupid as you might think.
> 
> But we disagree that the proper solution is to add an ad hoc hack which
> shortcuts stuff and creates a maintenance problem in the long run.
> 
> Just dismissing a proposal based on 'oh this adds more code to the idle
> path' and then yelling 'you all do not get it' is not going to solve this
> in any way.
> 
> You simply refuse to look at the big picture and you are just not willing
> to analyze the involved bits and pieces and find a solution which allows to
> serve both the NOHZ power saving and the interrupt driven workloads best.
> 
> All you are doing is providing data about the status quo, and then
> deferring from that, that you need an extra magic code path. That
> information is just a inventory of the current behaviour and the current
> behaviour is caused by the current implementation. There is nothing set in
> stone with that implementation and it can be changed.
> 
> But you are just in refusal mode and instead of sitting down and doing the
> hard work, you accuse other people that they are not realizing the problem
> and insist on your sloppy hackery. That's simply not the way it works.

Don't get me wrong, even if a fast path is acceptable, we still need to
figure out if the coming idle is short and when to switch. I'm just worried
about if irq timings is not an ideal statistics, we have to skip it too.

Let me a little time to digest the details of irq timings and obtain some
data to see if it's suitable for my case.

Thanks,
-Aubrey