From: Yu Zhao <yuzhao@google.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andi Kleen <ak@linux.intel.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
Jesse Barnes <jsbarnes@google.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Jonathan Corbet <corbet@lwn.net>,
Matthew Wilcox <willy@infradead.org>,
Mel Gorman <mgorman@suse.de>,
Michael Larabel <Michael@michaellarabel.com>,
Rik van Riel <riel@surriel.com>, Vlastimil Babka <vbabka@suse.cz>,
Will Deacon <will@kernel.org>, Ying Huang <ying.huang@intel.com>,
linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
page-reclaim@google.com, x86@kernel.org,
Konstantin Kharlamov <Hi-Angel@yandex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging
Date: Thu, 13 Jan 2022 02:43:38 -0700 [thread overview]
Message-ID: <Yd/0Sgxy+jLm5cqd@google.com> (raw)
In-Reply-To: <YdxEqFPLDf+wI0xX@dhcp22.suse.cz>
On Mon, Jan 10, 2022 at 03:37:28PM +0100, Michal Hocko wrote:
> On Sun 09-01-22 20:58:02, Yu Zhao wrote:
> > On Fri, Jan 07, 2022 at 10:00:31AM +0100, Michal Hocko wrote:
> > > On Fri 07-01-22 09:55:09, Michal Hocko wrote:
> > > [...]
> > > > > In this case, lru_gen_mm_walk is small (160 bytes); it's per direct
> > > > > reclaimer; and direct reclaimers rarely come here, i.e., only when
> > > > > kswapd can't keep up in terms of the aging, which is similar to the
> > > > > condition where the inactive list is empty for the active/inactive
> > > > > lru.
> > > >
> > > > Well, this is not a strong argument to be honest. Kswapd being stuck
> > > > and the majority of the reclaim being done in the direct reclaim
> > > > context is a situation I have seen many many times.
> > >
> > > Also do not forget that memcg reclaim is effectivelly only direct
> > > reclaim. Not that the memcg reclaim indicates a global memory shortage
> > > but it can add up and race with the global reclaim as well.
> >
> > I don't dispute any of the above, and I probably don't like this code
> > more than you do.
> >
> > But let's not forget the purposes of PF_MEMALLOC, besides preventing
> > recursive reclaims, include letting reclaim dip into reserves so that
> > it can make more free memory. So I think it's acceptable if the
> > following conditions are met:
> > 1. The allocation size is small.
> > 2. The number of allocations is bounded.
> > 3. Its failure doesn't stall reclaim.
> > And it'd be nice if
> > 4. The allocation happens rarely, e.g., slow path only.
>
> I would add
> 0. The allocation should be done only if absolutely _necessary_.
>
> Please keep in mind that whatever you allocate from that context will be
> consuming a very precious memory reserves which are shared with other
> components of the system. Even worse these can go all the way to
> depleting memory completely where other things can fall apart.
I agree but I also see a distinction:
1,2,3 are objective;
0,4 are subjective.
For some users, page reclaim itself could be not absolutely necessary
because they are okay with OOM kills. But for others, the situation
could be reversed.
> > The code in question meets all of them.
> >
> > 1. This allocation is 160 bytes.
> > 2. It's bounded by the number of page table walkers which, in the
> > worst, is same as the number of mm_struct's.
> > 3. Most importantly, its failure doesn't stall the aging. The aging
> > will fallback to the rmap-based function lru_gen_look_around().
> > But this function only gathers the accessed bit from at most 64
> > PTEs, meaning it's less efficient (retains ~80% performance gains).
> > 4. This allocation is rare, i.e., only when the aging is required,
> > which is similar to the low inactive case for the active/inactive
> > lru.
>
> I think this fallback behavior deserves much more detailed explanation
> in changelogs.
Will do.
> > The bottom line is I can try various optimizations, e.g., preallocate
> > a few buffers for a limited number of page walkers and if this number
> > has been reached, fallback to the rmap-based function. But I have yet
> > to see evidence that calls for additional complexity.
>
> I would disagree here. This is not an optimization. You should be
> avoiding allocations from the memory reclaim because any allocation just
> add a runtime behavior complexity and potential corner cases.
Would __GFP_NOMEMALLOC address your concern? It prevents allocations
from accessing the reserves even under PF_MEMALLOC.
next prev parent reply other threads:[~2022-01-13 9:43 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-04 20:22 [PATCH v6 0/9] Multigenerational LRU Framework Yu Zhao
2022-01-04 20:22 ` [PATCH v6 1/9] mm: x86, arm64: add arch_has_hw_pte_young() Yu Zhao
2022-01-05 10:45 ` Will Deacon
2022-01-05 20:47 ` Yu Zhao
2022-01-06 10:30 ` Will Deacon
2022-01-07 7:25 ` Yu Zhao
2022-01-11 14:19 ` Will Deacon
2022-01-11 22:27 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 2/9] mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG Yu Zhao
2022-01-04 21:24 ` Linus Torvalds
2022-01-04 20:22 ` [PATCH v6 3/9] mm/vmscan.c: refactor shrink_node() Yu Zhao
2022-01-04 20:22 ` [PATCH v6 4/9] mm: multigenerational lru: groundwork Yu Zhao
2022-01-04 21:34 ` Linus Torvalds
2022-01-11 8:16 ` Aneesh Kumar K.V
2022-01-12 2:16 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 5/9] mm: multigenerational lru: mm_struct list Yu Zhao
2022-01-07 9:06 ` Michal Hocko
2022-01-08 0:19 ` Yu Zhao
2022-01-10 15:21 ` Michal Hocko
2022-01-12 8:08 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 6/9] mm: multigenerational lru: aging Yu Zhao
2022-01-06 16:06 ` Michal Hocko
2022-01-06 21:27 ` Yu Zhao
2022-01-07 8:43 ` Michal Hocko
2022-01-07 21:12 ` Yu Zhao
2022-01-06 16:12 ` Michal Hocko
2022-01-06 21:41 ` Yu Zhao
2022-01-07 8:55 ` Michal Hocko
2022-01-07 9:00 ` Michal Hocko
2022-01-10 3:58 ` Yu Zhao
2022-01-10 14:37 ` Michal Hocko
2022-01-13 9:43 ` Yu Zhao [this message]
2022-01-13 12:02 ` Michal Hocko
2022-01-19 6:31 ` Yu Zhao
2022-01-19 9:44 ` Michal Hocko
2022-01-10 15:01 ` Michal Hocko
2022-01-10 16:01 ` Vlastimil Babka
2022-01-10 16:25 ` Michal Hocko
2022-01-11 23:16 ` Yu Zhao
2022-01-12 10:28 ` Michal Hocko
2022-01-13 9:25 ` Yu Zhao
2022-01-07 13:11 ` Michal Hocko
2022-01-07 23:36 ` Yu Zhao
2022-01-10 15:35 ` Michal Hocko
2022-01-11 1:18 ` Yu Zhao
2022-01-11 9:00 ` Michal Hocko
[not found] ` <1641900108.61dd684cb0e59@mail.inbox.lv>
2022-01-11 12:15 ` Michal Hocko
2022-01-11 14:22 ` Alexey Avramov
2022-01-07 14:44 ` Michal Hocko
2022-01-10 4:47 ` Yu Zhao
2022-01-10 10:54 ` Michal Hocko
2022-01-19 7:04 ` Yu Zhao
2022-01-19 9:42 ` Michal Hocko
2022-01-23 21:28 ` Yu Zhao
2022-01-24 14:01 ` Michal Hocko
2022-01-10 16:57 ` Michal Hocko
2022-01-12 1:01 ` Yu Zhao
2022-01-12 10:17 ` Michal Hocko
2022-01-12 23:43 ` Yu Zhao
2022-01-13 11:57 ` Michal Hocko
2022-01-23 21:40 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 7/9] mm: multigenerational lru: eviction Yu Zhao
2022-01-11 10:37 ` Aneesh Kumar K.V
2022-01-12 8:05 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 8/9] mm: multigenerational lru: user interface Yu Zhao
2022-01-10 10:27 ` Mike Rapoport
2022-01-12 8:35 ` Yu Zhao
2022-01-12 10:31 ` Michal Hocko
2022-01-12 15:45 ` Mike Rapoport
2022-01-13 9:47 ` Yu Zhao
2022-01-13 10:31 ` Aneesh Kumar K.V
2022-01-13 23:02 ` Yu Zhao
2022-01-14 5:20 ` Aneesh Kumar K.V
2022-01-14 6:50 ` Yu Zhao
2022-01-04 20:22 ` [PATCH v6 9/9] mm: multigenerational lru: Kconfig Yu Zhao
2022-01-04 21:39 ` Linus Torvalds
2022-01-04 20:22 ` [PATCH v6 0/9] Multigenerational LRU Framework Yu Zhao
2022-01-04 20:30 ` Yu Zhao
2022-01-04 21:43 ` Linus Torvalds
2022-01-05 21:12 ` Yu Zhao
2022-01-07 9:38 ` Michal Hocko
2022-01-07 18:45 ` Yu Zhao
2022-01-10 15:39 ` Michal Hocko
2022-01-10 22:04 ` Yu Zhao
2022-01-10 22:46 ` Jesse Barnes
2022-01-11 1:41 ` Linus Torvalds
2022-01-11 10:40 ` Michal Hocko
2022-01-11 8:41 ` Yu Zhao
2022-01-11 8:53 ` Holger Hoffstätte
2022-01-11 9:26 ` Jan Alexander Steffens (heftig)
2022-01-11 16:04 ` Shuang Zhai
2022-01-12 1:46 ` Suleiman Souhlal
2022-01-12 6:07 ` Sofia Trinh
2022-01-12 16:17 ` Daniel Byrne
2022-01-18 9:21 ` Yu Zhao
2022-01-18 9:36 ` Donald Carr
2022-01-19 20:19 ` Steven Barrett
2022-01-19 22:25 ` Brian Geffon
2022-01-05 2:44 ` Shuang Zhai
2022-01-05 8:55 ` SeongJae Park
2022-01-05 10:53 ` Yu Zhao
2022-01-05 11:25 ` SeongJae Park
2022-01-05 21:06 ` Yu Zhao
2022-01-10 14:49 ` Alexey Avramov
2022-01-11 10:24 ` Alexey Avramov
2022-01-12 20:56 ` Oleksandr Natalenko
2022-01-13 8:59 ` Yu Zhao
2022-01-23 5:43 ` Barry Song
2022-01-25 6:48 ` Yu Zhao
2022-01-28 8:54 ` Barry Song
2022-02-08 9:16 ` Yu Zhao
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