linux-kernel - Re: [PATCH V4] irq: Track the interrupt timings

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Message-ID: <alpine.LFD.2.20.1606141105320.1714@knanqh.ubzr>
Date:	Tue, 14 Jun 2016 11:10:17 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	Thomas Gleixner <tglx@...utronix.de>, shreyas@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	rafael@...nel.org, vincent.guittot@...aro.org
Subject: Re: [PATCH V4] irq: Track the interrupt timings

On Tue, 14 Jun 2016, Daniel Lezcano wrote:

> On 06/10/2016 04:52 PM, Thomas Gleixner wrote:
> 
> > > +	timings->sum -= timings->values[timings->w_index];
> > > +	timings->values[timings->w_index] = diff;
> > > +	timings->sum += diff;
> >
> > Now the real question is whether you really need all that math, checks and
> > memsets in the irq hotpath. If you make the storage slightly larger then you
> > can just store the values unconditionally in the circular buffer and do all
> > the computational stuff when you really it.
> 
> Yes, that was one concern when I wrote the code: do some basic computation
> when an interrupt occurs, and the rest after or do the entire math when
> entering idle.
> 
> If the storage is a bit larger (let's say 16 values) and there is no memset
> and the sum is not computed, at least we need a count for the number of values
> in the array before this one is fulfilled, otherwise the statistics will be
> wrong as we will take into account the entire array with old values, no ?

The point is not to change from 8 to 16 entries, but to store raw 64-bit 
timestamps instead of computed 32-bit deltas.  Whether or not those 
timestamps are too far apart and discarded can be done at idle entry 
time.


Nicolas