Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

From: Shakeel Butt <shakeelb@google.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>,
	Alexey Avramov <hakavlad@inbox.lv>,
	Rik van Riel <riel@surriel.com>, Mike Galbraith <efault@gmx.de>,
	Darrick Wong <djwong@kernel.org>,
	regressions@lists.linux.dev,
	Linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress
Date: Mon, 6 Dec 2021 23:14:58 -0800	[thread overview]
Message-ID: <CALvZod6NPzzD=rzvmgLNsudCDVNJWgwviijB1LztRAhCX7jQBA@mail.gmail.com> (raw)
In-Reply-To: <20211206112545.GF3366@techsingularity.net>

On Mon, Dec 6, 2021 at 3:25 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Sun, Dec 05, 2021 at 10:06:27PM -0800, Shakeel Butt wrote:
> > On Fri, Dec 3, 2021 at 11:08 AM Mel Gorman <mgorman@techsingularity.net> wrote:
> > >
> > [...]
> > > > I am in agreement with the motivation of the whole series. I am just
> > > > making sure that the motivation of VMSCAN_THROTTLE_NOPROGRESS based
> > > > throttle is more than just the congestion_wait of
> > > > mem_cgroup_force_empty_write.
> > > >
> > >
> > > The commit that primarily targets congestion_wait is 8cd7c588decf
> > > ("mm/vmscan: throttle reclaim until some writeback completes if
> > > congested"). The series recognises that there are other reasons why
> > > reclaim can fail to make progress that is not directly writeback related.
> > >
> >
> > I agree with throttling for VMSCAN_THROTTLE_[WRITEBACK|ISOLATED]
> > reasons. Please explain why we should throttle for
> > VMSCAN_THROTTLE_NOPROGRESS? Also 69392a403f49 claims "Direct reclaim
> > primarily is throttled in the page allocator if it is failing to make
> > progress.", can you please explain how?
>
> It could happen if the pages on the LRU are being reactivated continually
> or holding an elevated reference count for some reason (e.g. gup,
> page migration etc). The event is probably transient, hence the short
> throttling.
>

What's the worst that can happen if the kernel doesn't throttle at all
for these transient scenarios? Premature oom-kills? The kernel already
has some protection against such situations with retries i.e.
consecutive 16 unsuccessful reclaim tries have to fail to give up the
reclaim.

Anyways, I have shared my view which is 'no need to throttle at all
for no-progress reclaims for now and course correct if there are
complaints in future' but will not block the patch.

thanks,
Shakeel