RE: [PATCH 07/12] evtchn: cut short evtchn_reset()'s loop in the common case

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of Jan Beulich
> Sent: 28 September 2020 12:00
> To: xen-devel@lists.xenproject.org
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>; George Dunlap <George.Dunlap@eu.citrix.com>; Ian
> Jackson <iwj@xenproject.org>; Julien Grall <julien@xen.org>; Wei Liu <wl@xen.org>; Stefano Stabellini
> <sstabellini@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 <jbeulich@suse.com>

Reviewed-by: Paul Durrant <paul@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:
>