Re: [Xen-devel] [PATCH 5/5] xen: RCU: avoid busy waiting until the end of grace period.

[Julien Grall <julien.grall@xxxxxxx>]

Hi Dario,

On 27/07/2017 09:01, Dario Faggioli wrote:
> Instead of having the CPU where a callback is queued, busy
> looping on rcu_pending(), use a timer.
>
> In fact, we let the CPU go idla,e but we program a timer
> that will periodically wake it up, for checking whether the
> grace period has actually ended.
>
> It is kind of similar to introducing a periodic tick, but
> with a much more limited scope, and a lot less overhead. In
> fact, this timer is:
> - only active for the CPU(s) that have callbacks queued,
>   waiting for the end of a grace period;
> - only active when those CPU(s) are idle (and stopped as
>   soon as they resume execution).
If I read this correctly, it means on ARM the idling will now get 
interrupted periodically. This is a bit unfortunate, given that if you 
have a CPU doing nothing, you would still interrupt it intermittently.
I was expected that we could remove the CPU from the RCU whilst it is 
idle. Is there any reason for not doing that?
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel