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

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 08/04/2022 08:16, Jan Beulich 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.
"high" and "large" is quite vague. Do you have more details on where you 
observed this issue and the improvement after this patch?
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -1559,6 +1559,16 @@ void evtchn_move_pirqs(struct vcpu *v)
>       unsigned int port;
>       struct evtchn *chn;
>   
> +    /*
> +     * The work done below is an attempt to keep pIRQ-s on the pCPU-s that the
> +     * vCPU-s they're to be delivered to run on. In order to limit lock
> +     * contention, check for an empty list prior to acquiring the lock. In the
> +     * worst case a pIRQ just bound to this vCPU will be delivered elsewhere
> +     * until the vCPU is migrated (again) to another pCPU.
> +     */
AFAIU, the downside is another pCPU (and therefore vCPU) will get 
disturbed by the interrupt. Maybe we should revive "evtchn: convert 
domain event lock to an r/w one"?
> +    if ( !v->pirq_evtchn_head )
> +        return;
> +
>       spin_lock(&d->event_lock);
>       for ( port = v->pirq_evtchn_head; port; port = chn->u.pirq.next_port )
>       {
Cheers,

--
Julien Grall