Re: [PATCH RFC] evtchn: add early-out to evtchn_move_pirqs()

[Julien Grall <julien@xxxxxxx>]

Hi Luca,

On 08/04/2022 11:41, Luca Fancellu wrote:
> > On 8 Apr 2022, at 08:16, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >
> > See the code comment. The higher the rate of vCPU-s migrating across
> > pCPU-s, the less useful this attempted optimization actually is. With
> > credit2 the migration rate looks to be unduly high even on mostly idle
> > systems, and hence on large systems lock contention here isn't very
> > difficult to observe.
> >
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> I’ve tested this patch on a Juno board, starting Dom0, creating/destroying few 
> guests,
> doing some networking from guests of different cpu pool and everything worked.
FWIW, we don't support PIRQ on Arm (alloc_pirq_struct() will always 
return NULL). So evtchn_mode_pirqs() is basically a NOP.
This patch has the advantage for Arm to avoid taking the lock for 
nothing. I will comment on the x86 part separately.
Cheers,

--
Julien Grall