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Re: [PATCH RFC] evtchn: add early-out to evtchn_move_pirqs()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Fri, 8 Apr 2022 10:41:14 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>
- Delivery-date: Fri, 08 Apr 2022 10:42:28 +0000
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- Thread-topic: [PATCH RFC] evtchn: add early-out to evtchn_move_pirqs()
> 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.
Tested-by: Luca Fancellu <luca.fancellu@xxxxxxx>
>
> --- 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.
> + */
> + if ( !v->pirq_evtchn_head )
> + return;
> +
> spin_lock(&d->event_lock);
> for ( port = v->pirq_evtchn_head; port; port = chn->u.pirq.next_port )
> {
>
>
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