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SubjectRe: [PATCH 11/23] clocksource: atomic signals

* Daniel Walker <dwalker@mvista.com> wrote:

> > I see little difference between your and John's code: both poll
> > something - you poll an atomic "did a new clocksource arrive" flag
> > in the timer interrupt, John takes the clocksource_lock spinlock and
> > checks a "did a new clocksource arrive" variable. Both are global
> > atomic variables in essence.
>
> The original version has more operations on every timer interrupt.
> Also changing the spinlock to an atomic eliminates the possibility of
> contention in the timer interrupt ..

there is precisely /zero/ contention on the clocksource_lock! It is a
very short-held lock, and it's only held by the timer interrupt and some
really rare operations like 'clocksource register' or 'show
clocksources'.

> > what i'd see as a real cleanup here would be to get away from this
> > 'poll whether there's any clocksource update' model, and to just
> > ensure that a running timer irq will always see the latest
> > clocksource. I.e. to run the change_clocksource() logic (and the
> > following updates) when a new clock source is selected - not when
> > the next timer interrupt runs. That would propagate all effects of a
> > new clock source immediately.
>
> You could reduce the code in the interrupt handler (which is good),
> but I think you'll end up with a polling model regardless.. If you add
> some locking between the interrupt handler and something else you may
> as well add the run time of that new critical section to the timer
> latency . So I'm not sure it would be a outright win ..

I think you didnt understand what i said: the point is to /remove/ the
polling, and to replace it with a natural lock that is held anyway:
xtime_lock or whatever other exclusion mechanism. Again, there is almost
/never/ any contention on this lock so there's no 'latency to add'. But
the polling overhead in every timer irq, even if it's just a single
atomic flag, does add up in every timer tick.

you also didnt seem to understand my other point:

> > I.e. to run the change_clocksource() logic (and the following
> > updates) when a new clock source is selected - not when the next
> > timer interrupt runs. That would propagate all effects of a new
> > clock source immediately.

that is actually more important from a design cleanliness POV than the
basic avoidance of some polling overhead.

Ingo
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