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

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Date:	Tue, 14 Jun 2016 18:36:01 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	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, Nicolas Pitre wrote:
> On Tue, 14 Jun 2016, Daniel Lezcano wrote:
> > 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.

Correct, and you don't have to know how many timestamps are in the array
simply because if it is cleared at init time, then any not yet set value will
create a large gap, which you filter out.

The point is to make the fast path overhead as small as possible. And if
that's just a store and index increment, then it can be inline and not a
function call.

Thanks,

	tglx