Re: [PATCH] psi: annotate refault stalls from IO submission

[Johannes Weiner]

CCing Jens for bio layer stuff

On Tue, Jul 23, 2019 at 10:02:26AM +1000, Dave Chinner wrote:
> Even better: If this memstall and "refault" check is needed to
> account for bio submission blocking, then page cache iteration is
> the wrong place to be doing this check. It should be done entirely
> in the bio code when adding pages to the bio because we'll only ever
> be doing page cache read IO on page cache misses. i.e. this isn't
> dependent on adding a new page to the LRU or not - if we add a new
> page then we are going to be doing IO and so this does not require
> magic pixie dust at the page cache iteration level

That could work. I had it at the page cache level because that's
logically where the refault occurs. But PG_workingset encodes
everything we need from the page cache layer and is available where
the actual stall occurs, so we should be able to push it down.

> e.g. bio_add_page_memstall() can do the working set check and then
> set a flag on the bio to say it contains a memstall page. Then on
> submission of the bio the memstall condition can be cleared.

A separate bio_add_page_memstall() would have all the problems you
pointed out with the original patch: it's magic, people will get it
wrong, and it'll be hard to verify and notice regressions.

How about just doing it in __bio_add_page()? PG_workingset is not
overloaded - when we see it set, we can generally and unconditionally
flag the bio as containing userspace workingset pages.

At submission time, in conjunction with the IO direction, we can
clearly tell whether we are reloading userspace workingset data,
i.e. stalling on memory.

This?

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