Re: [PATCH] mm: Mark idle page tracking as BROKEN

From: SeongJae Park <sj38.park@gmail.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: akpm@linuxfoundation.org, linux-mm@kvack.org,
	Heiko Carstens <hca@linux.ibm.com>,
	Rafael Aquini <aquini@redhat.com>,
	Vlastimil Babka <vbabka@suse.cz>, Yu Zhao <yuzhao@google.com>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	kirill.shutemov@linux.intel.com, amit@kernel.org
Subject: Re: [PATCH] mm: Mark idle page tracking as BROKEN
Date: Mon, 14 Jun 2021 13:49:26 +0000	[thread overview]
Message-ID: <20210614134926.17279-1-sjpark@amazon.de> (raw)
In-Reply-To: <20210612000714.775825-1-willy@infradead.org>

From: SeongJae Park <sjpark@amazon.de>

Hello Matthew,

On Sat, 12 Jun 2021 01:07:14 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> In discussion with other MM developers around how idle page tracking
> should be fixed for transparent huge pages, several expressed the opinion
> that it should be removed as it is inefficient at accomplishing the
> job that it is supposed to, and we have better mechanisms (eg uffd) for
> accomplishing the same goals these days.

I think the THP case[1] is an intended behavior[2].  Could you please share a
link to the discussion or a detailed summary if possible?

> 
> Mark the feature as BROKEN for now and we can remove it entirely in a
> few months if nobody complains.  It is not enabled by Android, ChromeOS,
> Debian, Fedora or SUSE.  Red Hat enabled it with RHEL-8.1 and UEK followed
> suit, but I have been unable to find why RHEL enabled it.

Amazon Linux is also using it[3], for DAMON[4].  In detail, DAMON doesn't use
Idle Page Tracking but PG_Idle in kernel space, to avoid interfering the
reclaim logic[5].  So, I'm ok with removing the Idle Page Tracking user space
interface, but gonna be opposed to removing PG_Idle.

Nevertheless, the interference is not a real problem to DAMON, as DAMON is
aimed to provide just a reasonable quality of the monitoring, rather than
strict correctness.  Hence, if people think the interference is also not a
problem for the reclaim logic (after all, it does nothing unless sysadmin
manually turns it on in runtime, and can be turned off at anytime), I would
simply update DAMON code to don't use PG_Idle, add warnings in the doc, and
wouldn't be opposed to this change.

But, if not, I think this change will be a big problem to us.  If the problem
is a lack of dedicated maintainer for Idle Page Tracking or PG_Idle, I can
volunteer.

[1] https://lore.kernel.org/linux-mm/YMGKVmt8trMJ9kOP@casper.infradead.org/
[2] https://lore.kernel.org/linux-mm/20210614081610.16123-1-sjpark@amazon.de/
[3] https://github.com/amazonlinux/linux/blob/amazon-5.10.y/master/mm/damon/Kconfig#L19
[4] https://lore.kernel.org/linux-mm/20210520075629.4332-1-sj38.park@gmail.com/
[5] https://lore.kernel.org/linux-mm/20210525154427.30921-1-sjpark@amazon.de/

Thanks,
SeongJae Park

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