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

[Jeff Moyer <jmoyer@redhat.com>]

Christoph Hellwig <hch@infradead.org> writes:

> 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..

I think Jens' bug report is exactly that explanation, no?  To avoid
priority inversion?

-Jeff