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[2003:cb:c706:4e00:31ad:5274:e21c:b59]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b367c01f6sm15094953f8f.23.2024.07.30.09.11.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Jul 2024 09:11:51 -0700 (PDT) Message-ID: <95ed1631-ff62-4627-8dc6-332096e673b4@redhat.com> Date: Tue, 30 Jul 2024 18:11:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/6] mm: split underutilized THPs To: Usama Arif , akpm@linux-foundation.org, linux-mm@kvack.org Cc: hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, roman.gushchin@linux.dev, yuzhao@google.com, baohua@kernel.org, ryan.roberts@arm.com, rppt@kernel.org, willy@infradead.org, cerasuolodomenico@gmail.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20240730125346.1580150-1-usamaarif642@gmail.com> <3cd1b07d-7b02-4d37-918a-5759b23291fb@gmail.com> <73b97a03-3742-472f-9a36-26ba9009d715@gmail.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 1tug7htbhyiyx79xs5msyocjj1qmes4f X-Rspamd-Queue-Id: 42843A0019 X-Rspamd-Server: rspam11 X-HE-Tag: 1722355916-178644 X-HE-Meta: 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 XBPG7/w/ BP++S9Pk4IN8yb+zFgFHc9YW+/Y4iE8wc4A9rTc84udqBkpOxv1g9b7hNdZGAmdx7tGXZ1GnUxh3xOb6kP1/NSyizQ1ts9gAKzdhBoDV1zLrrRZAtxM6DLfdHu//LJ8bgftx2RUhbrxypxKBYdIYLa2xuYsjdv9Fk8NCcgb4Ox6J2J+uJVMHt1eYAyiWN59rk6F2+6MBrC95s1A7cp/ljkXj5+M2GlHpHOY3WOEJvoK01ZmLB/EWyC/jDiKqHVoRXS8Eu3vNZuPRzaYZmLxnKA1Be454FkoXQEgvFzKM9ztDKvcPYF49jISI0aL8KtW/UKbF8xqerDEYAsiIHaZeSob7767M3GZks+kJM+IHYlqTheWCYuNcnvUnk7ZB/1S3NVVMSqIgYcjVXx/TWA/XzhZoBB/gNMZ2QGzuHgf7ASveFXSMgy3PiA/8naQJ4y81Va/w+C3uXnGJQqzGm3fZ3zpSmRg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000008, 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 30.07.24 17:19, Usama Arif wrote: > > > On 30/07/2024 16:14, Usama Arif wrote: >> >> >> On 30/07/2024 15:35, David Hildenbrand wrote: >>> On 30.07.24 14:45, Usama Arif wrote: >>>> The current upstream default policy for THP is always. However, Meta >>>> uses madvise in production as the current THP=always policy vastly >>>> overprovisions THPs in sparsely accessed memory areas, resulting in >>>> excessive memory pressure and premature OOM killing. >>>> Using madvise + relying on khugepaged has certain drawbacks over >>>> THP=always. Using madvise hints mean THPs aren't "transparent" and >>>> require userspace changes. Waiting for khugepaged to scan memory and >>>> collapse pages into THP can be slow and unpredictable in terms of performance >>>> (i.e. you dont know when the collapse will happen), while production >>>> environments require predictable performance. If there is enough memory >>>> available, its better for both performance and predictability to have >>>> a THP from fault time, i.e. THP=always rather than wait for khugepaged >>>> to collapse it, and deal with sparsely populated THPs when the system is >>>> running out of memory. >>>> >>>> This patch-series is an attempt to mitigate the issue of running out of >>>> memory when THP is always enabled. During runtime whenever a THP is being >>>> faulted in or collapsed by khugepaged, the THP is added to a list. >>>> Whenever memory reclaim happens, the kernel runs the deferred_split >>>> shrinker which goes through the list and checks if the THP was underutilized, >>>> i.e. how many of the base 4K pages of the entire THP were zero-filled. >>>> If this number goes above a certain threshold, the shrinker will attempt >>>> to split that THP. Then at remap time, the pages that were zero-filled are >>>> not remapped, hence saving memory. This method avoids the downside of >>>> wasting memory in areas where THP is sparsely filled when THP is always >>>> enabled, while still providing the upside THPs like reduced TLB misses without >>>> having to use madvise. >>>> >>>> Meta production workloads that were CPU bound (>99% CPU utilzation) were >>>> tested with THP shrinker. The results after 2 hours are as follows: >>>> >>>>                              | THP=madvise |  THP=always   | THP=always >>>>                              |             |               | + shrinker series >>>>                              |             |               | + max_ptes_none=409 >>>> ----------------------------------------------------------------------------- >>>> Performance improvement     |      -      |    +1.8%      |     +1.7% >>>> (over THP=madvise)          |             |               | >>>> ----------------------------------------------------------------------------- >>>> Memory usage                |    54.6G    | 58.8G (+7.7%) |   55.9G (+2.4%) >>>> ----------------------------------------------------------------------------- >>>> max_ptes_none=409 means that any THP that has more than 409 out of 512 >>>> (80%) zero filled filled pages will be split. >>>> >>>> To test out the patches, the below commands without the shrinker will >>>> invoke OOM killer immediately and kill stress, but will not fail with >>>> the shrinker: >>>> >>>> echo 450 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none >>>> mkdir /sys/fs/cgroup/test >>>> echo $$ > /sys/fs/cgroup/test/cgroup.procs >>>> echo 20M > /sys/fs/cgroup/test/memory.max >>>> echo 0 > /sys/fs/cgroup/test/memory.swap.max >>>> # allocate twice memory.max for each stress worker and touch 40/512 of >>>> # each THP, i.e. vm-stride 50K. >>>> # With the shrinker, max_ptes_none of 470 and below won't invoke OOM >>>> # killer. >>>> # Without the shrinker, OOM killer is invoked immediately irrespective >>>> # of max_ptes_none value and kill stress. >>>> stress --vm 1 --vm-bytes 40M --vm-stride 50K >>>> >>>> Patches 1-2 add back helper functions that were previously removed >>>> to operate on page lists (needed by patch 3). >>>> Patch 3 is an optimization to free zapped tail pages rather than >>>> waiting for page reclaim or migration. >>>> Patch 4 is a prerequisite for THP shrinker to not remap zero-filled >>>> subpages when splitting THP. >>>> Patches 6 adds support for THP shrinker. >>>> >>>> (This patch-series restarts the work on having a THP shrinker in kernel >>>> originally done in >>>> https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@fb.com/. >>>> The THP shrinker in this series is significantly different than the >>>> original one, hence its labelled v1 (although the prerequisite to not >>>> remap clean subpages is the same).) >>> >>> As shared previously, there is one issue with uffd (even when currently not active for a VMA!), where we must not zap present page table entries. >>> >>> Something that is always possible (assuming no GUP pins of course, which) is replacing the zero-filled subpages by shared zeropages. >>> >>> Is that being done in this patch set already, or are we creating pte_none() entries? >>> >> >> I think thats done in Patch 4/6. In function try_to_unmap_unused, we have below which I think does what you are suggesting? i.e. point to shared zeropage and not clear pte for uffd armed vma. >> >> if (userfaultfd_armed(pvmw->vma)) { >> newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), >> pvmw->vma->vm_page_prot)); >> ptep_clear_flush(pvmw->vma, pvmw->address, pvmw->pte); >> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); >> } > > > Ah are you suggesting userfaultfd_armed(pvmw->vma) will evaluate to false even if its uffd? I think something like below would work in that case. I remember one ugly case in QEMU with postcopy live-migration where we must not zap zero-filled pages. I am not 100% regarding THP (if it could be enabled at that point), but imagine the following 1) mmap(), enable THP 2) Migrate a bunch of pages from the source during precopy (writing to the memory). Might end up creating THPs (during fault/khugepaged) 3) Register UFFD on the VMA 4) Disable new THPs from forming via MADV_NOHUGEPAGE on the VMA 5) Discard any pages that have been re-dirtied or not migrated yet 6) Migrate-on-demand any holes using uffd If we discard zero-filled pages between 2) and 3) we might get wrong uffd notifications in 6 for pages that have already been migrated). I'll have to check if that actually happens in that sequence in QEMU: if QEMU would disable THP right before 2) we would be safe. But I recall that it is not the case :/ -- Cheers, David / dhildenb