Re: [PATCH v6 8/9] mm: multigenerational lru: user interface

From: Mike Rapoport <rppt@kernel•org>
To: Yu Zhao <yuzhao@google•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>,
	Michal Hocko <mhocko@kernel•org>, 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 8/9] mm: multigenerational lru: user interface
Date: Wed, 12 Jan 2022 17:45:40 +0200	[thread overview]
Message-ID: <Yd73pDkMOMVHhXzu@kernel.org> (raw)
In-Reply-To: <Yd6S6Js1W4AnFFmv@google.com>

On Wed, Jan 12, 2022 at 01:35:52AM -0700, Yu Zhao wrote:
> On Mon, Jan 10, 2022 at 12:27:19PM +0200, Mike Rapoport wrote:
> > Hi,
> > 
> > On Tue, Jan 04, 2022 at 01:22:27PM -0700, Yu Zhao wrote:
> > > Add /sys/kernel/mm/lru_gen/enabled as a runtime kill switch.
> > > 
> > > Add /sys/kernel/mm/lru_gen/min_ttl_ms for thrashing prevention.
> > > Compared with the size-based approach, e.g., [1], this time-based
> > > approach has the following advantages:
> > > 1) It's easier to configure because it's agnostic to applications and
> > >    memory sizes.
> > > 2) It's more reliable because it's directly wired to the OOM killer.
> > > 
> > > Add /sys/kernel/debug/lru_gen for working set estimation and proactive
> > > reclaim. Compared with the page table-based approach and the PFN-based
> > > approach, e.g., mm/damon/[vp]addr.c, this lruvec-based approach has
> > > the following advantages:
> > > 1) It offers better choices because it's aware of memcgs, NUMA nodes,
> > >    shared mappings and unmapped page cache.
> > > 2) It's more scalable because it's O(nr_hot_evictable_pages), whereas
> > >    the PFN-based approach is O(nr_total_pages).
> > > 
> > > Add /sys/kernel/debug/lru_gen_full for debugging.
> > > 
> > > [1] https://lore.kernel.org/lkml/20211130201652.2218636d@mail.inbox.lv/
> > > 
> > > Signed-off-by: Yu Zhao <yuzhao@google•com>
> > > Tested-by: Konstantin Kharlamov <Hi-Angel@yandex•ru>
> > > ---
> > >  Documentation/vm/index.rst        |   1 +
> > >  Documentation/vm/multigen_lru.rst |  62 +++++
> > 
> > The description of user visible interfaces should go to
> > Documentation/admin-guide/mm
> > 
> > Documentation/vm/multigen_lru.rst should have contained design description
> > and the implementation details and it would be great to actually have such
> > document.
> 
> Will do, thanks.
> 
> > >  include/linux/nodemask.h          |   1 +
> > >  mm/vmscan.c                       | 415 ++++++++++++++++++++++++++++++
> > >  4 files changed, 479 insertions(+)
> > >  create mode 100644 Documentation/vm/multigen_lru.rst
> > > 
> > > diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst
> > > index 6f5ffef4b716..f25e755b4ff4 100644
> > > --- a/Documentation/vm/index.rst
> > > +++ b/Documentation/vm/index.rst
> > > @@ -38,3 +38,4 @@ algorithms.  If you are looking for advice on simply allocating memory, see the
> > >     unevictable-lru
> > >     z3fold
> > >     zsmalloc
> > > +   multigen_lru
> > > diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst
> > > new file mode 100644
> > > index 000000000000..6f9e0181348b
> > > --- /dev/null
> > > +++ b/Documentation/vm/multigen_lru.rst
> > > @@ -0,0 +1,62 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +=====================
> > > +Multigenerational LRU
> > > +=====================
> > > +
> > > +Quick start
> > > +===========
> > > +Runtime configurations
> > > +----------------------
> > > +:Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the
> > > + feature wasn't enabled by default.
> > 
> > Required for what? This sentence seem to lack context. Maybe add an
> > overview what is Multigenerational LRU so that users will have an idea what
> > these knobs control.
> 
> Apparently I left an important part of this quick start in the next
> patch, where Kconfig options are added. I'm wonder whether I should
> squash the next patch into this one.

I think documentation deserves a separate patch.

-- 
Sincerely yours,
Mike.

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