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Message-ID: <20221122162747.opnte2csfr5hors2@google.com>
Date: Tue, 22 Nov 2022 16:27:47 +0000
From: Shakeel Butt <shakeelb@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Matthew Wilcox <willy@...radead.org>,
David Hildenbrand <david@...hat.com>,
Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
Yang Shi <shy828301@...il.com>,
John Hubbard <jhubbard@...dia.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Sidhartha Kumar <sidhartha.kumar@...cle.com>,
Muchun Song <songmuchun@...edance.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
Mina Almasry <almasrymina@...gle.com>,
James Houghton <jthoughton@...gle.com>,
"Zach O'Keefe" <zokeefe@...gle.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH 0/3] mm,thp,rmap: rework the use of subpages_mapcount
On Mon, Nov 21, 2022 at 09:16:58AM -0800, Linus Torvalds wrote:
> On Mon, Nov 21, 2022 at 8:59 AM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > Is there a plan to remove lock_page_memcg() altogether which I missed? I
> > am planning to make lock_page_memcg() a nop for cgroup-v2 (as it shows
> > up in the perf profile on exit path)
>
> Yay. It seems I'm not the only one hating it.
>
> > but if we are removing it then I should just wait.
>
> Well, I think Johannes was saying that at least the case I disliked
> (the rmap removal from the page table tear-down - I strongly suspect
> it's the one you're seeing on your perf profile too)
Yes indeed that is the one.
- 99.89% 0.00% fork-large-mmap [kernel.kallsyms] [k] entry_SYSCALL_64_after_hw◆
entry_SYSCALL_64_after_hwframe
- do_syscall_64
- 48.94% __x64_sys_exit_group
do_group_exit
- do_exit
- 48.94% exit_mm
mmput
- __mmput
- exit_mmap
- 48.61% unmap_vmas
- 48.61% unmap_single_vma
- unmap_page_range
- 48.60% zap_p4d_range
- 44.66% zap_pte_range
+ 12.61% tlb_flush_mmu
- 9.38% page_remove_rmap
2.50% lock_page_memcg
2.37% unlock_page_memcg
0.61% PageHuge
4.80% vm_normal_page
2.56% __tlb_remove_page_size
0.85% lock_page_memcg
0.53% PageHuge
2.22% __tlb_remove_page_size
0.93% vm_normal_page
0.72% page_remove_rmap
> can be removed
> entirely as long as it's done under the page table lock (which my
> final version of the rmap delaying still was).
>
> See
>
> https://lore.kernel.org/all/Y2llcRiDLHc2kg%2FN@cmpxchg.org/
>
> for his preliminary patch.
>
> That said, if you have some patch to make it a no-op for _other_
> reasons, and could be done away with _entirely_ (not just for rmap),
> then that would be even better.
I am actually looking at deprecating the whole "move charge"
funcitonality of cgroup-v1 i.e. the underlying reason lock_page_memcg
exists. That already does not work for couple of cases like partially
mapped THP and madv_free'd pages. Though that deprecation process would
take some time. In the meantime I was looking at if we can make these
functions nop for cgroup-v2.
thanks,
Shakeel
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