[libvirt] PATCH: 6/25: Avoid event loop wakups in remote driver

[Jim Meyering]

"Daniel P. Berrange" <berrange at redhat.com> wrote:
> When going into the call() method, we temporarily disable the
> event wakup callback. The call() method is already processing
> any async messages which arrive, so letting the main event loop
> wakup here is inefficient, and indeed will block the main event
> loop on the mutex while the main thread is in call(). We really
> don't want that !
>
>  remote_internal.c |   42 ++++++++++++++++++++++++++++++++++--------
>  1 file changed, 34 insertions(+), 8 deletions(-)
>
> Daniel
>
> diff --git a/src/remote_internal.c b/src/remote_internal.c
...
> +doCall (virConnectPtr conn, struct private_data *priv,
...
> +static int
> +call (virConnectPtr conn, struct private_data *priv,
> +      int flags /* if we are in virConnectOpen */,
> +      int proc_nr,
> +      xdrproc_t args_filter, char *args,
> +      xdrproc_t ret_filter, char *ret)
> +{
> +    int rv;
> +    /*
> +     * Avoid needless wake-ups of the event loop in the
> +     * case where this call is being made from a different
> +     * thread than the event loop. These wake-ups would
> +     * cause the event loop thread to be blocked on the
> +     * mutex for the duration of the call
> +     */
> +    if (priv->watch >= 0)
> +        virEventUpdateHandle(priv->watch, 0);
> +
> +    rv = doCall(conn, priv,flags, proc_nr,
> +                args_filter, args,
> +                ret_filter, ret);
> +
> +    if (priv->watch >= 0)
> +        virEventUpdateHandle(priv->watch, VIR_EVENT_HANDLE_READABLE);

Looks fine.  ACK.

At first I wondered if someday the incoming event-update-handle
type might be something other than VIR_EVENT_HANDLE_READABLE,
in which case you'd want to save the original before
doCall, and restore it afterward.  Probably won't happen.