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[2003:d8:2f19:9c00:568:7df7:e1:293d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-451f82871a4sm42742905e9.0.2025.06.06.10.37.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Jun 2025 10:37:40 -0700 (PDT) Message-ID: <4c1d5033-0c90-4672-84a1-15978ced245d@redhat.com> Date: Fri, 6 Jun 2025 19:37:39 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mm: khugepaged: use largest enabled hugepage order for min_free_kbytes To: Usama Arif , Andrew Morton , linux-mm@kvack.org Cc: hannes@cmpxchg.org, shakeel.butt@linux.dev, riel@surriel.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, hughd@google.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20250606143700.3256414-1-usamaarif642@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.06.25 16:37, Usama Arif wrote: > On arm64 machines with 64K PAGE_SIZE, the min_free_kbytes and hence the > watermarks are evaluated to extremely high values, for e.g. a server with > 480G of memory, only 2M mTHP hugepage size set to madvise, with the rest > of the sizes set to never, the min, low and high watermarks evaluate to > 11.2G, 14G and 16.8G respectively. > In contrast for 4K PAGE_SIZE of the same machine, with only 2M THP hugepage > size set to madvise, the min, low and high watermarks evaluate to 86M, 566M > and 1G respectively. > This is because set_recommended_min_free_kbytes is designed for PMD > hugepages (pageblock_order = min(HPAGE_PMD_ORDER, PAGE_BLOCK_ORDER)). > Such high watermark values can cause performance and latency issues in > memory bound applications on arm servers that use 64K PAGE_SIZE, eventhough > most of them would never actually use a 512M PMD THP. > > Instead of using HPAGE_PMD_ORDER for pageblock_order use the highest large > folio order enabled in set_recommended_min_free_kbytes. > With this patch, when only 2M THP hugepage size is set to madvise for the > same machine with 64K page size, with the rest of the sizes set to never, > the min, low and high watermarks evaluate to 2.08G, 2.6G and 3.1G > respectively. When 512M THP hugepage size is set to madvise for the same > machine with 64K page size, the min, low and high watermarks evaluate to > 11.2G, 14G and 16.8G respectively, the same as without this patch. > > An alternative solution would be to change PAGE_BLOCK_ORDER by changing > ARCH_FORCE_MAX_ORDER to a lower value for ARM64_64K_PAGES. However, this > is not dynamic with hugepage size, will need different kernel builds for > different hugepage sizes and most users won't know that this needs to be > done as it can be difficult to detmermine that the performance and latency > issues are coming from the high watermark values. > > All watermark numbers are for zones of nodes that had the highest number > of pages, i.e. the value for min size for 4K is obtained using: > cat /proc/zoneinfo | grep -i min | awk '{print $2}' | sort -n | tail -n 1 | awk '{print $1 * 4096 / 1024 / 1024}'; > and for 64K using: > cat /proc/zoneinfo | grep -i min | awk '{print $2}' | sort -n | tail -n 1 | awk '{print $1 * 65536 / 1024 / 1024}'; > > An arbirtary min of 128 pages is used for when no hugepage sizes are set > enabled. > > Signed-off-by: Usama Arif > --- > include/linux/huge_mm.h | 25 +++++++++++++++++++++++++ > mm/khugepaged.c | 32 ++++++++++++++++++++++++++++---- > mm/shmem.c | 29 +++++------------------------ > 3 files changed, 58 insertions(+), 28 deletions(-) > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h > index 2f190c90192d..fb4e51ef0acb 100644 > --- a/include/linux/huge_mm.h > +++ b/include/linux/huge_mm.h > @@ -170,6 +170,25 @@ static inline void count_mthp_stat(int order, enum mthp_stat_item item) > } > #endif > > +/* > + * Definitions for "huge tmpfs": tmpfs mounted with the huge= option > + * > + * SHMEM_HUGE_NEVER: > + * disables huge pages for the mount; > + * SHMEM_HUGE_ALWAYS: > + * enables huge pages for the mount; > + * SHMEM_HUGE_WITHIN_SIZE: > + * only allocate huge pages if the page will be fully within i_size, > + * also respect madvise() hints; > + * SHMEM_HUGE_ADVISE: > + * only allocate huge pages if requested with madvise(); > + */ > + > + #define SHMEM_HUGE_NEVER 0 > + #define SHMEM_HUGE_ALWAYS 1 > + #define SHMEM_HUGE_WITHIN_SIZE 2 > + #define SHMEM_HUGE_ADVISE 3 > + > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > > extern unsigned long transparent_hugepage_flags; > @@ -177,6 +196,12 @@ extern unsigned long huge_anon_orders_always; > extern unsigned long huge_anon_orders_madvise; > extern unsigned long huge_anon_orders_inherit; > > +extern int shmem_huge __read_mostly; > +extern unsigned long huge_shmem_orders_always; > +extern unsigned long huge_shmem_orders_madvise; > +extern unsigned long huge_shmem_orders_inherit; > +extern unsigned long huge_shmem_orders_within_size; Do really all of these have to be exported? > + > static inline bool hugepage_global_enabled(void) > { > return transparent_hugepage_flags & > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 15203ea7d007..e64cba74eb2a 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2607,6 +2607,26 @@ static int khugepaged(void *none) > return 0; > } > > +static int thp_highest_allowable_order(void) Did you mean "largest" ? > +{ > + unsigned long orders = READ_ONCE(huge_anon_orders_always) > + | READ_ONCE(huge_anon_orders_madvise) > + | READ_ONCE(huge_shmem_orders_always) > + | READ_ONCE(huge_shmem_orders_madvise) > + | READ_ONCE(huge_shmem_orders_within_size); > + if (hugepage_global_enabled()) > + orders |= READ_ONCE(huge_anon_orders_inherit); > + if (shmem_huge != SHMEM_HUGE_NEVER) > + orders |= READ_ONCE(huge_shmem_orders_inherit); > + > + return orders == 0 ? 0 : fls(orders) - 1; > +} But how does this interact with large folios / THPs in the page cache? > + > +static unsigned long min_thp_pageblock_nr_pages(void) Reading the function name, I have no idea what this function is supposed to do. -- Cheers, David / dhildenb