Re: [PATCH] cfq: priority boost on meta/prio marked IO

Hi, Jens,

Jens Axboe [off-list ref] writes:

At Facebook, we have a number of cases where people use ionice to set a
lower priority, then end up having tasks stuck for a long time because
eg meta data updates from an idle priority tasks is blocking out higher
priority processes. It's bad enough that it will trigger the softlockup
warning.

I expect a higher prio process could be blocked on a lower prio process
reading the same metadata, too.  I had a hard time tracking down where
REQ_META WRITE I/O was issued outside of the journal or writeback paths
(and I hope you're not ionice-ing those!).  Eventually, with the help of
sandeen, I found some oddball cases that I doubt you're running into.
Can you enlighten me as to where this (REQ_META write I/O) is happening?
I don't disagree that it's a problem, I just would like to understand
your problem case better.

Anyway, it seems to me you could just set REQ_PRIO in the code paths you
care about instead of modifying CFQ to treat REQ_META and REQ_PRIO as
the same thing, which essentially undoes commit 65299a3b788bd ("block:
separate priority boosting from REQ_META") from Christoph.

Thanks!
Jeff