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Date:   Fri, 3 Apr 2020 17:15:34 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "Anna.Schumaker@Netapp.com" <Anna.Schumaker@Netapp.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, linux-mm@kvack.org,
        linux-nfs@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE
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On Thu 02-04-20 10:53:20, Neil Brown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> loop block driver, where a daemon needs to write to one bdi in
> order to free up writes queued to another bdi.
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> This patch changes the heuristic to ignore the global limits and
> consider only the limit relevant to the bdi being written to.  This
> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> should not introduce surprises.  This has the desired result of
> protecting the task from the consequences of large amounts of dirty data
> queued for other devices.

While I understand that you want to have per bdi throttling for those
"special" files I am still missing how this is going to provide the
additional room that the additnal 25% gave them previously. I might
misremember or things have changed (what you mention as non-trivial
heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
forward progress. Care to expan some more on how this is handled now?
Maybe we do not need it anymore but calling that out explicitly would be
really helpful.
-- 
Michal Hocko
SUSE Labs