From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Thomas Gleixner <tglx@linutronix.de>
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 11:29:52 -0400 (EDT) [thread overview]
Message-ID: <alpine.LFD.2.20.1606101128180.1714@knanqh.ubzr> (raw)
In-Reply-To: <alpine.DEB.2.11.1606101716530.5839@nanos>
On Fri, 10 Jun 2016, Thomas Gleixner wrote:
> 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.
Given your later argument I agree.
> > > > + */
> > > > + 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 ?
Go figure. This code has come a long way.
> > > 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
>
next prev parent reply other threads:[~2016-06-10 15:35 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-13 11:05 [PATCH V4] irq: Track the interrupt timings 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
2016-06-10 15:29 ` Nicolas Pitre [this message]
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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