On 4/12/21 5:20 PM, Gaetan Rivet wrote:
> Similar to what was done for the PMD threads [1], reduce the performance
> impact of quiescing too often in the offload thread.
>
> After each processed offload, the offload thread currently quiesce and
> will sync with RCU. This synchronization can be lengthy and make the
> thread unnecessary slow.
>
> Instead attempt to quiesce every 10 ms at most. While the queue is
> empty, the offload thread remains quiescent.
>
> [1]: 81ac8b3b194c ("dpif-netdev: Do RCU synchronization at fixed interval
> in PMD main loop.")
>
> Signed-off-by: Gaetan Rivet <[email protected]>
> Reviewed-by: Eli Britstein <[email protected]>
> ---
> lib/dpif-netdev.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index d458bcb12..4e403a9e5 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -2740,15 +2740,20 @@ err_free:
> return -1;
> }
>
> +#define DP_NETDEV_OFFLOAD_QUIESCE_INTERVAL_US (10 * 1000) /* 10 ms */
> +
> static void *
> dp_netdev_flow_offload_main(void *data OVS_UNUSED)
> {
> struct dp_offload_thread_item *offload;
> struct ovs_list *list;
> long long int latency_us;
> + long long int next_rcu;
> + long long int now;
> const char *op;
> int ret;
>
> + next_rcu = time_usec() + DP_NETDEV_OFFLOAD_QUIESCE_INTERVAL_US;
> for (;;) {
> ovs_mutex_lock(&dp_offload_thread.mutex);
> if (ovs_list_is_empty(&dp_offload_thread.list)) {
> @@ -2756,6 +2761,7 @@ dp_netdev_flow_offload_main(void *data OVS_UNUSED)
> ovs_mutex_cond_wait(&dp_offload_thread.cond,
> &dp_offload_thread.mutex);
> ovsrcu_quiesce_end();
> + next_rcu = time_usec() + DP_NETDEV_OFFLOAD_QUIESCE_INTERVAL_US;
> }
> list = ovs_list_pop_front(&dp_offload_thread.list);
> dp_offload_thread.enqueued_item--;
> @@ -2779,7 +2785,9 @@ dp_netdev_flow_offload_main(void *data OVS_UNUSED)
> OVS_NOT_REACHED();
> }
>
> - latency_us = time_usec() - offload->timestamp;
> + now = time_usec();
> +
> + latency_us = now - offload->timestamp;
> mov_avg_cma_update(&dp_offload_thread.cma, latency_us);
> mov_avg_ema_update(&dp_offload_thread.ema, latency_us);
>
> @@ -2787,7 +2795,13 @@ dp_netdev_flow_offload_main(void *data OVS_UNUSED)
> ret == 0 ? "succeed" : "failed", op,
> UUID_ARGS((struct uuid *) &offload->flow->mega_ufid));
> dp_netdev_free_flow_offload(offload);
> - ovsrcu_quiesce();
> +
> + /* Do RCU synchronization at fixed interval. */
> + if (now > next_rcu) {
> + if (!ovsrcu_try_quiesce()) {
> + next_rcu += DP_NETDEV_OFFLOAD_QUIESCE_INTERVAL_US;
Commit 81ac8b3b194c do it a bit differently, by using current time as a
base for calculating the next deadline while you use current deadline
(which is in the past) to calculate the next one.
Under heavy load, won't it just end up doing the RCU synchronization at
every it iteration, as next_rcu would always run behind?
> + }
> + }
> }
>
> return NULL;
>
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