linux-kernel - Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

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Date:	Mon, 03 Feb 2014 08:17:47 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Morten Rasmussen <morten.rasmussen@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Len Brown <len.brown@...el.com>,
	Preeti Murthy <preeti.lkml@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC PATCH 3/3] idle: store the idle state index in the struct
 rq

On 2/3/2014 6:56 AM, Peter Zijlstra wrote:
>
> Arjan, could you have a look at teaching your Thunderpants to wrap lines
> at ~80 chars please?

I'll try but it suffers from Apple-disease


>> 1) A latency driven one
>> 2) A performance impact on
>>
>> first one is pretty much the exit latency related time, sort of a
>> "expected time to first instruction" (currently menuidle has the
>> 99.999% worst case number, which is not useful for this, but is a
>> first approximation). This is obviously the dominating number for
>> expected-short running tasks
>>
>> second on is more of a "is there any cache/TLB left or is it flushed"
>> kind of metric. It's more tricky to compute, since what is the cost of
>> an empty cache (or even a cache migration) after all....  .... but I
>> suspect it's in part what the scheduler will care about more for
>> expected-long  running tasks.
>
> Yeah, so currently we 'assume' cache hotness based on runtime; see
> task_hot(). A hint that the CPU wiped its caches might help there.

if there's a simple api like

sched_cpu_cache_wiped(int llc)

that would be very nice for this; the menuidle side knows this
for some cases and thus can just call it. This would be a very
small and minimal change

* if you don't care about llc vs core local caches then that
   parameter can go away

* I assume this is also called for the local cpu... if not then we
   need to add a cpu number argument

* we can also call this from architecture code when wbinvd or the
   arm equivalent is called etc



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