linux-kernel - Re: [RFC][PATCH] perf: Rewrite enabled/running timekeeping

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Date:   Mon, 4 Sep 2017 17:56:06 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Vince Weaver <vince@...ter.net>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH] perf: Rewrite enabled/running timekeeping

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.

> 
> 
>