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

[Peter Zijlstra]

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? On ctx switch we should stop iteration for a PMU
once we fail to schedule 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.