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SubjectRe: [RFC][PATCH] perf: Rewrite enabled/running timekeeping
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On 04.09.2017 15:08, Peter Zijlstra wrote:
> On Mon, Sep 04, 2017 at 01:46:45PM +0300, Alexey Budankov wrote:
>>> So the below completely rewrites timekeeping (and probably breaks
>>> world) but does away with the need to touch events that don't get
>>> scheduled.
>>
>> We still need and do iterate thru all events at some points e.g. on context switches.
>
> Why do we _need_ to?

We do so in the current implementation with several tstamp_* fields.

> On ctx switch we should stop iteration for a PMU once we fail toschedule an event, same as for rotation>
>>> The basic idea is really simple, we have a single timestamp and
>>> depending on the state we update enabled/running. This obviously only
>>> requires updates when we change state and when we need up-to-date
>>> timestamps (read).
>>
>> I would prefer to have this rework in a FSM similar to that below,
>> so state transition and the corresponding tstamp, total_time_enabled
>> and total_time_running manipulation logic would be consolidated in
>> one place and adjacent lines of code.
>>
>> From the table below event->state FSM is not as simple as it may seem
>> on the first sight so in order to avoid regressions after rework we
>> better keep that in mind and explicitly implement allowed and disallowed
>> state transitions.
>
> Maybe if we introduce something like CONFIG_PERF_DEBUG, but I fear that
> for normal operation that's all fairly horrible overhead.
>
>> A I O E X D U
>>
>> A Te+,Tr+ Te+,Tr+ Te+,Tr+ Te+,Tr+ Te+,Tr+ Te+,Tr+ ---
>> ts ts ts ts ts ts
>>
>> I Te+,ts Te+,ts Te+,ts Te+,ts Te+,ts Te+,ts ---
>>
>> O Te=0,Tr=0, Te=0,Tr=0, Te=0,Tr=0 Te=0,Tr=0 Te=0,Tr=0 Te=0,Tr=0 ---
>> ts ts ts ts ts ts
>>
>> E Te=0,Tr=0, Te=0,Tr=0, Te=0,Tr=0 Te=0,Tr=0 Te=0,Tr=0 Te=0,Tr=0 ---
>> ts ts ts ts ts ts
>>
>> X --- --- --- --- --- --- ---
>>
>> D --- --- --- --- --- --- ---
>>
>> U --- Te=0,Tr=0 Te=0,Tr=0 --- --- --- ---
>> ts ts
>>
>> LEGEND:
>>
>> U - allocation, A - ACTIVE, I - INACTIVE, O - OFF,
>> E - ERROR, X - EXIT, D - DEAD,
>
> Not sure we care about the different <0 values, they're all effectively
> OFF.

We still need to care about proper initial state of timings when moving above >=0 state.

>
>
>

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