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Tue, 01 Jul 2025 19:19:37 -0700 (PDT) Message-ID: <0b316beb-0c49-444f-983c-e8a8a3e76dfc@gmail.com> Date: Wed, 2 Jul 2025 10:19:30 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.15.0 Subject: Re: [PATCH] mm/mglru: Stop try_to_inc_min_seq() if the oldest generation LRU lists are not empty To: Yuanchu Xie Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, yuzhao@google.com, kinseyho@google.com, david@redhat.com, mhocko@kernel.org, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Jia References: <20250630080603.36171-1-jiahao.kernel@gmail.com> From: Hao Jia In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2025/7/2 08:31, Yuanchu Xie wrote: Sorry, I got my email wrong. I'll reply again to make sure the kernel mail lists can receive it. > On Mon, Jun 30, 2025 at 1:06 AM Hao Jia wrote: >> >> From: Hao Jia >> >> In try_to_inc_min_seq(), if the oldest generation of LRU lists >> (anonymous and file) are not empty. Then we should return directly >> to avoid unnecessary subsequent overhead. >> >> Corollary: If the lrugen->folios[gen][type][zone] lists of both >> anonymous and file are not empty, try_to_inc_min_seq() will fail. >> >> Proof: Taking LRU_GEN_ANON as an example, consider the following two cases: >> >> Case 1: min_seq[LRU_GEN_ANON] <= seq (seq is lrugen->max_seq - MIN_NR_GENS) >> >> Since min_seq[LRU_GEN_ANON] has not increased, >> so min_seq[LRU_GEN_ANON] is still equal to lrugen->min_seq[LRU_GEN_ANON]. >> Therefore, in the following judgment: >> min_seq[LRU_GEN_ANON] <= lrugen->min_seq[LRU_GEN_ANON] is always true. >> So, we will not increase the seq of the oldest generation of anonymous, >> and try_to_inc_min_seq() will return false. >> >> case 2: min_seq[LRU_GEN_ANON] > seq (seq is lrugen->max_seq - MIN_NR_GENS) >> >> If min_seq[LRU_GEN_ANON] > seq, that is, lrugen->min_seq[LRU_GEN_ANON] > seq > This part doesn't make sense to me. > The code is as follows: > > /* find the oldest populated generation */ > for_each_evictable_type(type, swappiness) { > while (min_seq[type] + MIN_NR_GENS <= lrugen->max_seq) { > gen = lru_gen_from_seq(min_seq[type]); > > for (zone = 0; zone < MAX_NR_ZONES; zone++) { > if (!list_empty(&lrugen->folios[gen][type][zone])) > goto next; > } > > min_seq[type]++; > } > > Here, it could be that , min_seq[type] > lrugen->max_seq - MIN_NR_GENS > (what you refer to as seq) > However, this is a result of incrementing a copy of > lrugen->min_seq[type] as this piece of code finds the oldest populated > generation. > Hi, Yuanchu Sorry for the confusion. I am assuming that if the oldest generation LRU lists (anonymous and file) are not empty, in other words, *min_seq[type]* has not increased. The above part has been executed, and it is known that min_seq[type] has not increased（that is, min_seq[type]=lrugen->min_seq[type] at this time）, so the rest of the reasoning. Maybe you mean that under the above premise min_seq[type] is impossible to be greater than seq (seq is lrugen->max_seq - MIN_NR_GENS)? If so, case2 does not need to be discussed and reasoned. In either case, my patch will work well. Thanks, Hao > next: > ; > } > >> Then min_seq[LRU_GEN_ANON] is assigned seq. > This is not necessarily true, because swappiness can be 0, and the > assignments happen to prevent one LRU type from going more than 1 gen > past the other. > so if `min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] == seq` is > true, then min_seq[LRU_GEN_ANON] is not assigned seq. > Yes, if min_seq[LRU_GEN_ANON] is not assigned seq, then the situation is the same as case 1. min_seq[LRU_GEN_ANON] is equal to lrugen->min_seq[LRU_GEN_ANON]. in the following judgment: min_seq[LRU_GEN_ANON] <= lrugen->min_seq[LRU_GEN_ANON] is always true. Case 2 wants to discuss another situation, that is, when min_seq[LRU_GEN_ANON] is assigned to seq. The following judgment is whether min_seq[LRU_GEN_ANON] <= lrugen->min_seq[LRU_GEN_ANON] is always true. > >> Therefore, in the following judgment: >> min_seq[LRU_GEN_ANON] (seq) <= lrugen->min_seq[LRU_GEN_ANON] is always true. >> So, we will not update the oldest generation seq of anonymous, >> and try_to_inc_min_seq() will return false. >> >> It is similar for LRU_GEN_FILE. Therefore, in try_to_inc_min_seq(), >> if the oldest generation LRU lists (anonymous and file) are not empty, >> in other words, min_seq[type] has not increased. >> we can directly return false to avoid unnecessary checking overhead later. > Yeah I don't think this proof holds. If you think it does please > elaborate more and make your assumptions more clear. > >> >> Signed-off-by: Hao Jia >> --- >> mm/vmscan.c | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index f8dfd2864bbf..3ba63d87563f 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -3928,6 +3928,7 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness) >> int gen, type, zone; >> bool success = false; >> struct lru_gen_folio *lrugen = &lruvec->lrugen; >> + int seq_inc_flags[ANON_AND_FILE] = {0}; >> DEFINE_MIN_SEQ(lruvec); >> >> VM_WARN_ON_ONCE(!seq_is_valid(lruvec)); >> @@ -3943,11 +3944,20 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness) >> } >> >> min_seq[type]++; >> + seq_inc_flags[type] = 1; >> } >> next: >> ; >> } >> >> + /* >> + * If the oldest generation of LRU lists (anonymous and file) >> + * are not empty, we can directly return false to avoid unnecessary >> + * checking overhead later. >> + */ >> + if (!seq_inc_flags[LRU_GEN_ANON] && !seq_inc_flags[LRU_GEN_FILE]) >> + return success; >> + >> /* see the comment on lru_gen_folio */ >> if (swappiness && swappiness <= MAX_SWAPPINESS) { >> unsigned long seq = lrugen->max_seq - MIN_NR_GENS; >> -- >> 2.34.1 >> >> > I don't understand what problem this patch tries to solve. > > Yuanchu My pathch is that if we already know that min_seq[type] (including anonymous and file) has not increased, we can directly let try_to_inc_min_seq() return failure to reduce unnecessary checking overhead later. After my above reasoning, this does not change the original behavior of try_to_inc_min_seq(). I added some code to count the number of try_to_inc_min_seq() calls and the number of times the situation mentioned in my patch is hit. Run the test in tools/testing/selftests/cgroup/test_memcontrol on my machine. hit_cnt: 1215 total_cnt: 1702 The hit rate is about 71% Thanks, Hao