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Subject: Re: [PATCH] psi: annotate refault stalls from IO submission
To:     Johannes Weiner <hannes@cmpxchg.org>,
        Dave Chinner <david@fromorbit.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190722201337.19180-1-hannes@cmpxchg.org>
 <20190723000226.GV7777@dread.disaster.area>
 <20190723190438.GA22541@cmpxchg.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <2d80cfdb-f5e0-54f1-29a3-a05dee5b94eb@kernel.dk>
Date:   Tue, 23 Jul 2019 13:34:50 -0600
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On 7/23/19 1:04 PM, Johannes Weiner wrote:
> 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?

Not vehemently opposed to it, even if it sucks having to test page flags
in the hot path. Maybe even do:

	if (!bio_flagged(bio, BIO_WORKINGSET) && PageWorkingset(page))
		bio_set_flag(bio, BIO_WORKINGSET);

to at least avoid it for the (common?) case where multiple pages are
marked as workingset.

-- 
Jens Axboe