LU-8515 osc: Send RPCs when extents are full

LU-8515 osc: Send RPCs when extents are full

Currently, Lustre decides to send an RPC under a number of
conditions (such as memory pressure or lock cancellcation);
one of the conditions it looks for is "enough dirty pages
to fill an RPC". This worked fine when only one process
could be dirtying pages at a time, but in newer Lustre
versions, more than one process can write to the same
file (and the same osc object) at once.

In this case, the "count dirty pages method" will see there
are enough dirty pages to fill an RPC, but since the dirty
pages are being created by multiple writers, they are not
contiguous and will not fit in to one RPC. This resulted in
many RPCs of less than full size being sent, despite a
good I/O pattern. (Earlier versions of Lustre usually
send only full RPCs when presented with this pattern.)

Instead, we remove this check and add extents to a special
full extent list when they reach max pages per RPC, then
send from that list. (This is similar to high priority
and urgent extents.)

With a good I/O pattern, like usually used in benchmarking,
it should be possible to send only full size RPCs. This
patch achieves that without degrading performance in other
cases.

In IOR tests with multiple writers to a single file,
this patch improves performance by several times, and
returns performance to equal levels (single striped files)
or much greater levels (very high speed OSTs, files
with many stripes) vs earlier versions.

Supporting data is provided in LU-8515.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Icc3c99b953d11f774600996648c9b20ed1f81f89
Reviewed-on: https://review.whamcloud.com/22012
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>