Re: [PATCH net-next] net: openvswitch: don't schedule rebalancing if there are no datapaths

[Aaron Conole <aconole@redhat.com>]

Ilya Maximets <i.maximets@ovn.org> writes:

> During namespace initialization the masks rebalancing work is
> scheduled and automatically re-scheduled every 4 seconds afterwards.
> This is happening in every namespace.  On a large kubernetes node with
> 500 pods, i.e., 500+ namespaces, this creates a decent amount of
> unnecessary churn scheduling 500 jobs every 4 seconds that take the
> mutex, check that there are no datapaths in their namespace, release
> the mutex, re-schedule themselves and exit.  These 500 unnecessary
> mutex locks may hold off operations in a single namespace that
> actually has a datapath configured and has real user requests to
> handle under this lock.  They can also add delay to removal of other
> namespaces as ovs_exit_net() needs to take that lock as well and
> synchronously waits for the work to be cancelled.
>
> Let's only fire the job when the first datapath is actually created
> and not re-arm it if there are no more datapaths configured in the
> namespace.
>
> Another approach would be to make ovs_mutex per-namespace, but it's
> a much larger change that should be handled separately, and the
> unnecessary work scheduling feels like a waste regardless.
>
> It's safe to check and re-arm outside of the mutex as DP_CMD_NEW
> handler will re-arm if the new datapath appears.  The scheduling
> attempt also doesn't change the work or delay if it is already queued,
> so it's also safe to call multiple times.
>
> Skipping the re-arming is more elegant than canceling on removal of
> the last datapath as it allows us to not think about potential race
> conditions at a negligible cost of potentially one extra re-scheduling.
>
> msecs_to_jiffies() moved to the macro to save on line length.
>
> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
> ---

Reviewed-by: Aaron Conole <aconole@redhat.com>