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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 09, 2016 at 11:55:56AM -0400, Jeff Moyer wrote:
> 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.

XFS does lots of REQ_META writes from _xfs_buf_ioapply().  But none
of those should be in the critical path as the all metadata is logged
first and then written back later.

> 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.

And I'm still waiting for someone to explain when exactly REQ_PRIO
should be used..