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From: Thomas Gleixner <tglx@linutronix.de>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>,
	shreyas@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
	peterz@infradead.org, rafael@kernel.org,
	vincent.guittot@linaro.org
Subject: Re: [PATCH V4] irq: Track the interrupt timings
Date: Fri, 10 Jun 2016 17:20:43 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.11.1606101716530.5839@nanos> (raw)
In-Reply-To: <alpine.LFD.2.20.1606101059350.1714@knanqh.ubzr>

On Fri, 10 Jun 2016, Nicolas Pitre wrote:
> On Fri, 10 Jun 2016, Thomas Gleixner wrote:
> > > +	diff = now - prev;
> > > +
> > > +	/*
> > > +	 * microsec (actually 1024th of a milisec) precision is good
> > > +	 * enough for our purpose.
> > > +	 */
> > > +	diff >>= 10;
> > 
> > And that shift instruction is required because of the following?
> > 
> > >   	 * Otherwise we know the magnitude of diff is
> > > +	 * well within 32 bits.
> > 
> > AFAICT that's pointless. You are not saving anything because NSEC_PER_SEC is
> > smaller than 2^32 and your 8 values are not going to overflow 64 bit in the
> > sum.
> 
> Those values are squared later, so we really want 32 bits here.

Well, you can do sum >> 10 exaclty once when you calculate stuff.
 
> > > +	 */
> > > +	if (unlikely(diff > USEC_PER_SEC)) {
> > > +		memset(timings, 0, sizeof(*timings));
> > > +		timings->timestamp = now;
> > 
> > Redundant store.
> 
> We just trashed all our data with the memset so the current timestamp 
> needs to be restored.

So why doing a full memset and not only on the array ?

> > 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.
> 
> Well... given that you need an IRQ everytime you come out of idle that 
> means there will always be more IRQs than entries into idle, so you're 
> probably right.

Glad you agree.

Thanks,

	tglx

  reply	other threads:[~2016-06-10 15:22 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-13 11:05 Daniel Lezcano
2016-04-22 18:21 ` Daniel Lezcano
2016-05-03 13:44 ` Daniel Lezcano
2016-06-10 14:52 ` Thomas Gleixner
2016-06-10 15:11   ` Nicolas Pitre
2016-06-10 15:20     ` Thomas Gleixner [this message]
2016-06-10 15:29       ` Nicolas Pitre
2016-06-14 13:36   ` Daniel Lezcano
2016-06-14 15:10     ` Nicolas Pitre
2016-06-14 15:38       ` Daniel Lezcano
2016-06-14 16:36       ` Thomas Gleixner

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