[0/3] block: Fix IO priority mess

Message

Jan Kara June 1, 2022, 2:51 p.m. UTC

Hello,

recently I've been looking into 10% regression reported by our performance
measurement infrastructure in reaim benchmark that was bisected down to
5a9d041ba2f6 ("block: move io_context creation into where it's needed"). This
didn't really make much sense and it took me a while to understand this but the
culprit is actually in even older commit e70344c05995 ("block: fix default IO
priority handling") and 5a9d041ba2f6 just made the breakage visible.
Essentially the problem was that after these commits some IO was queued with IO
priority class IOPRIO_CLASS_BE while other IO was queued with IOPRIO_CLASS_NONE
and as a result they could not be merged together resulting in performance
regression. I think what commit e70344c05995 ("block: fix default IO
priority handling") did is actually broken not only because of this performance
regression but because of other reasons as well (see changelog of patch 3/3
for details) so this patch set aims at fixing it.

								Honza