> -----Original Message-----
> From: Xen-devel <xen-devel-boun...@lists.xenproject.org> On Behalf Of Jan
> Beulich
> Sent: 28 September 2020 12:00
> To: xen-devel@lists.xenproject.org
> Cc: Andrew Cooper <andrew.coop...@citrix.com>; George Dunlap
> <george.dun...@eu.citrix.com>; Ian
> Jackson <i...@xenproject.org>; Julien Grall <jul...@xen.org>; Wei Liu
> <w...@xen.org>; Stefano Stabellini
> <sstabell...@kernel.org>
> Subject: [PATCH 07/12] evtchn: cut short evtchn_reset()'s loop in the common
> case
>
> The general expectation is that there are only a few open ports left
> when a domain asks its event channel configuration to be reset.
> Similarly on average half a bucket worth of event channels can be
> expected to be inactive. Try to avoid iterating over all channels, by
> utilizing usage data we're maintaining anyway.
>
> Signed-off-by: Jan Beulich <jbeul...@suse.com>
Reviewed-by: Paul Durrant <p...@xen.org>
>
> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -232,7 +232,11 @@ void evtchn_free(struct domain *d, struc
> evtchn_port_clear_pending(d, chn);
>
> if ( consumer_is_xen(chn) )
> + {
> write_atomic(&d->xen_evtchns, d->xen_evtchns - 1);
> + /* Decrement ->xen_evtchns /before/ ->active_evtchns. */
> + smp_wmb();
> + }
> write_atomic(&d->active_evtchns, d->active_evtchns - 1);
>
> /* Reset binding to vcpu0 when the channel is freed. */
> @@ -1073,6 +1077,19 @@ int evtchn_unmask(unsigned int port)
> return 0;
> }
>
> +static bool has_active_evtchns(const struct domain *d)
> +{
> + unsigned int xen = read_atomic(&d->xen_evtchns);
> +
> + /*
> + * Read ->xen_evtchns /before/ active_evtchns, to prevent
> + * evtchn_reset() exiting its loop early.
> + */
> + smp_rmb();
> +
> + return read_atomic(&d->active_evtchns) > xen;
> +}
> +
> int evtchn_reset(struct domain *d, bool resuming)
> {
> unsigned int i;
> @@ -1097,7 +1114,7 @@ int evtchn_reset(struct domain *d, bool
> if ( !i )
> return -EBUSY;
>
> - for ( ; port_is_valid(d, i); i++ )
> + for ( ; port_is_valid(d, i) && has_active_evtchns(d); i++ )
> {
> evtchn_close(d, i, 1);
>
> @@ -1340,6 +1357,10 @@ int alloc_unbound_xen_event_channel(
>
> spin_unlock_irqrestore(&chn->lock, flags);
>
> + /*
> + * Increment ->xen_evtchns /after/ ->active_evtchns. No explicit
> + * barrier needed due to spin-locked region just above.
> + */
> write_atomic(&ld->xen_evtchns, ld->xen_evtchns + 1);
>
> out:
>
