[Bugs] [Bug 1547888] New: [brick-mux] incorrect event-thread scaling in server_reconfigure()

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1547888

            Bug ID: 1547888
           Summary: [brick-mux] incorrect event-thread scaling in
                    server_reconfigure()
           Product: GlusterFS
           Version: mainline
         Component: core
          Assignee: bugs at gluster.org
          Reporter: mchangir at redhat.com
                CC: amukherj at redhat.com, bugs at gluster.org,
                    moagrawa at redhat.com, rgowdapp at redhat.com



Description of problem:
Whenever BRICK_ATTACH or BRICK_TERMINATE management operation is triggered on
the brick, glusterfs_autoscale_threads() scales the event-threads correctly.
However, the subsequent xlator_reconfigure()s that are invoked pass a default
value of "event-threads" option and value to server_reconfigure() causing a
rescaling of event-threads to 2. This is because the value of "event-threads"
option is always 1, presuming that the "server.event-threads" wasn't set to a
different value.

eg.
In case of brick-mux, event-threads are scaled by +1 for BRICK_ATTACH and
scaled by -1 for BRICK_TERMINATE. So starting with 5 bricks, if 5 more bricks
are added in a single operation, the brick receives 5 BRICK_ATTACH messages
causing the event-threads to be incrementally scaled to 10. However, the xlator
reconfigures that get invoked causes the system default value for
"event-thread" option to be used causing the scaling of event-threads back to
2.

This may cause an unwanted fop throttling at the bricks.
Care needs to be taken so that the event thread count is not reset after
rescaling at BRICK_ATTACH/BRICK_TERMINATE management op.

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