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

[Arjan van de Ven <arjan@linux.intel.com>]

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