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

From: Yu Zhao <yuzhao@google.com>
To: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	akpm@linuxfoundation.org, Linux-MM <linux-mm@kvack.org>,
	 Heiko Carstens <hca@linux.ibm.com>,
	Rafael Aquini <aquini@redhat.com>,
	 Vladimir Davydov <vdavydov.dev@gmail.com>,
	 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	 Donald Dutile <ddutile@redhat.com>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm: Mark idle page tracking as BROKEN
Date: Wed, 16 Jun 2021 13:23:22 -0600	[thread overview]
Message-ID: <CAOUHufbdF=LBFZbJuJg+P73uoaR1jrC4zXSCsivT6F6puaDYFg@mail.gmail.com> (raw)
In-Reply-To: <7b35885b-1413-5e08-3930-c8c4b66bcfe7@redhat.com>

On Wed, Jun 16, 2021 at 2:43 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 16.06.21 10:36, Vlastimil Babka wrote:
> > On 6/16/21 8:22 AM, Yu Zhao wrote:
> >> On Tue, Jun 15, 2021 at 8:55 PM Matthew Wilcox <willy@infradead.org> wrote:
> >>>
> >>>
> >>> I don't know.  I asked the others on the call and the answer I got was
> >>> essentially "Just delete it".
> >>>
> >>> I'm kind of hoping the others speak up.
> >>
> >> I listed a couple of things when acking this patch. Being broken is
> >> not a problem as long as there are users who care about it. What made
> >> me think such users may not exist is that nobody ever complained about
> >> those things until we stumbled on them -- I'm not insisting on
> >> deleting this feature, just clarifying why I thought so.
> >
> > Similar feelings here. On the call it looked like the feature was abandoned by
> > its creators, and it wasn't clear if the distros that had it enabled did so due
> > to reasons that still apply for future versions. Sending the proposal and
> > getting a feedback that there are users is one of the expected valid outcomes.
>
> For us (RH) it will be very interesting to know the exact things that
> are "suboptimal" (I'm avoiding the terminology "broken" here), so we can
> actually evaluate if this might affect customers and might be worth
> "improving".

I consider the examples I gave in my first email breakages -- others
broke/break the idle page tracking -- and I think it's safe to assume
they will continue to happen.

If you are really looking for improvements, the page compaction has
always been a good example. For the idle page tracking, with physical
memory as little as 4GB, it needs to go thru one million PFNs, no
matter how many compound or buddy pages there are. For THPs, it will
try to get_page_unless_zero() on tail pages, which always fails. This
is why we discussed it in the meeting.

What can't be improved is the memory locality of PFNs. They are not
grouped by memcgs or processes. Two PFNs next to each other can be
from two processes with two sets of five-level page tables. The cache
misses simply outweigh any potential benefits one might get from this
feature, speaking as one of the customers.