[Xen-devel] Re: linux c/s 561

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Someone reported they could spin over twenty times before a soon-to-occur
jiffy 'tick' (specifically, the 'jiffies+1' tick) actually occurred. Which
makes sense since the 'slop' in Xen's timer_softirq_action() is 50us and the
cost of block-plus-wakeup-on-VIRQ_TIMER is about a couple of microseconds.
So that's over twenty times setting a timeout less than 50us in the future,
blocking, immediately waking up in the timer ISR, doing no work, setting a
timeout... etc.

So the patch avoids this little corner case. It's not a major issue, and the
side effect is that you may wait a little longer to do timer work.

 -- Keir

On 2/6/08 08:46, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Keir,
> 
> this change looks a little odd to me - the code as it was before already
> set the timeout to jiffies + 1 as the lowest possible value. So I don't
> understand how you could have observed any spinning over more than
> really short periods of time (where you happen to just see jiffies
> increment while setting the timeout).
> 
> I'd therefore appreciate any insight to the background of that change.
> 
> Thanks, Jan
> 



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