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Sat, 29 Nov 2025 01:24:13 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Sat, 29 Nov 2025 17:24:01 +0800 X-Gm-Features: AWmQ_blmdrt31__-OOWXQLljQ7HLIb-Qw0tzeo_DfLbYqFpVa7OT7qu7j6DxAoI Message-ID: Subject: Re: [PATCH 3/3] mm: optimize free_area_empty() check using per-migratetype counts To: Hongru Zhang Cc: akpm@linux-foundation.org, vbabka@suse.cz, david@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, rppt@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, Hongru Zhang Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B584018000B X-Stat-Signature: hq84tkfty9fekxr9et9orhgrjq99kbxt X-Rspam-User: X-HE-Tag: 1764408254-917190 X-HE-Meta: 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 MrqaHKZI ar4kn++JqpHNMU+x3MQ7r/4fttUwLIvpfut8L4rlz53T4mXKzeFEQKz7rYScAxg/BpLRt0v6wU5JK7IXqLPwZk/5I8ROTcjsgGeBPg8ipmazKk54u2DoDtyLy9jvxQsQMyc0aAktKZ+Xh1gJzbp7WJ83FkQJddWgIREqa+txo/UjJPoAD2l2a/ZUPkIiPYsg7t0Jt6e8ZMzVDlw1L9XEXeb2gmZvpK7FuYq8/AZA2q9aWgr6fqcoe5oeFfSDoTj7xqNLl+Q1LFyNOFRhNm+SXCBtD2lJ8guk1I4fNoH/b8wtxinll2Axel/xi5Da2ZSDiOfVHcP31nOqFRcgM7ZGSDhTdnf64pNDNl9XrjBuUgHSV/wJ/dOH0CTakmj6N5YFPgNxt5ZV1NZSEy99+CAO1fZ5xd5+59TDlS1cmP2tZCZ8ZWrhzhkrU6cFcYDVz2rfXUOqrI8U00E4iX8fjea3JDgFyZeZ3RymhSfCMJZ1ghQjRDrc= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Nov 29, 2025 at 8:04=E2=80=AFAM Barry Song <21cnbao@gmail.com> wrot= e: > > On Fri, Nov 28, 2025 at 11:13=E2=80=AFAM Hongru Zhang wrote: > > > > From: Hongru Zhang > > > > Use per-migratetype counts instead of list_empty() helps reduce a > > few cpu instructions. > > > > Signed-off-by: Hongru Zhang > > --- > > mm/internal.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/internal.h b/mm/internal.h > > index 1561fc2ff5b8..7759f8fdf445 100644 > > --- a/mm/internal.h > > +++ b/mm/internal.h > > @@ -954,7 +954,7 @@ int find_suitable_fallback(struct free_area *area, = unsigned int order, > > > > static inline bool free_area_empty(struct free_area *area, int migrate= type) > > { > > - return list_empty(&area->free_list[migratetype]); > > + return !READ_ONCE(area->mt_nr_free[migratetype]); > > I'm not quite sure about this. Since the counter is written and read more > frequently, cache coherence traffic may actually be higher than for the l= ist > head. > > I'd prefer to drop this unless there is real data showing it performs bet= ter. If the goal is to optimize free_area list checks and list_add, a reasonable approach is to organize the data structure to reduce false sharing between different mt and order entries. struct mt_free_area { struct list_head free_list; unsigned long nr_free; } ____cacheline_aligned; struct free_area { struct mt_free_area mt_free_area[MIGRATE_TYPES]; }; However, without supporting data, it=E2=80=99s unclear if the space increas= e is justified :-) Thanks Barry