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Wed, 18 Dec 2024 12:00:16 +0800 (CST) Message-ID: Date: Wed, 18 Dec 2024 12:00:15 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V7] mm, compaction: don't use ALLOC_CMA for unmovable allocations To: Johannes Weiner Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, 21cnbao@gmail.com, david@redhat.com, baolin.wang@linux.alibaba.com, vbabka@suse.cz, liuzixing@hygon.cn References: <1734436004-1212-1-git-send-email-yangge1116@126.com> <20241217155551.GA37530@cmpxchg.org> <93cf1aee-70df-426f-a3c0-1db8068bd59a@126.com> <20241218032936.GB37530@cmpxchg.org> From: Ge Yang In-Reply-To: <20241218032936.GB37530@cmpxchg.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:qSkvCgD3_yLPSGJnBtcfCg--.12633S2 X-Coremail-Antispam: 1Uf129KBjvJXoW3XrW5Xr1Dur43WF1furW5Awb_yoW7WF4Dpr W7ZFnFkws8XFy5Krs7tw1v9a4Ygw4rJF4UXw10vr1kurnI9F9FkF4DtFyUCFyUXr15JayY qFWUuF9rZF45Z3JanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07U6MKZUUUUU= X-Originating-IP: [118.242.3.34] X-CM-SenderInfo: 51dqwwjhrrila6rslhhfrp/1tbiOhq5G2diQIGEAAABsD X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 4DFC48000E X-Rspam-User: X-Stat-Signature: d9u138jxrqmdbb9s4nuejd9bzs4wi7sd X-HE-Tag: 1734494418-422382 X-HE-Meta: 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 CS7rSHnh cwWwV9kjh5Zn+akihbJTDK100rFBWBc4IOLw0WM13croOhnROWQwysJyOs2CIrB14xkhrhu7fkkBqO0bMaEYJuKXJTI7xUzzMArSatnknSCth0kKkCYRbgCXSY4R9PGwuJY7x8BjB6KRirwlIWQSTWjnj9aViBZEdeZhx+5KhmMa04xWaUixnE0+1XzlEMtAjwYvn36vRKVHG18D8GyX0cRKFIT/dS3UDvX94oumf7UVgGL09q2azRP1zfzCpjnotBcOQ9GjEktJz8LceF01mXEKjBjvvtU269PuSU9PaHjkxeIti3IfCrVtdiBcyFdVkakeEMnm/bkHjCeDesJICXhmIznYv/LGo2KFNORyPfC02CydXm3c5lbP1Z4VzB3RPjIm9JO4j/hMFC1wvMdD98nMMuQ4ZiMUyMJjkakwd2v1AvsXMzRvGuHBtTuJ9c0FcOZxQoG5omw2khGSK2NdxI/DSx6i0ljVUTs/h 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: 在 2024/12/18 11:29, Johannes Weiner 写道: > On Wed, Dec 18, 2024 at 10:15:06AM +0800, Ge Yang wrote: >> >> >> 在 2024/12/17 23:55, Johannes Weiner 写道: >>> Hello Yangge, >>> >>> On Tue, Dec 17, 2024 at 07:46:44PM +0800, yangge1116@126.com wrote: >>>> From: yangge >>>> >>>> Since commit 984fdba6a32e ("mm, compaction: use proper alloc_flags >>>> in __compaction_suitable()") allow compaction to proceed when free >>>> pages required for compaction reside in the CMA pageblocks, it's >>>> possible that __compaction_suitable() always returns true, and in >>>> some cases, it's not acceptable. >>>> >>>> There are 4 NUMA nodes on my machine, and each NUMA node has 32GB >>>> of memory. I have configured 16GB of CMA memory on each NUMA node, >>>> and starting a 32GB virtual machine with device passthrough is >>>> extremely slow, taking almost an hour. >>>> >>>> During the start-up of the virtual machine, it will call >>>> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory. >>>> Long term GUP cannot allocate memory from CMA area, so a maximum >>>> of 16 GB of no-CMA memory on a NUMA node can be used as virtual >>>> machine memory. Since there is 16G of free CMA memory on the NUMA >>>> node, watermark for order-0 always be met for compaction, so >>>> __compaction_suitable() always returns true, even if the node is >>>> unable to allocate non-CMA memory for the virtual machine. >>>> >>>> For costly allocations, because __compaction_suitable() always >>>> returns true, __alloc_pages_slowpath() can't exit at the appropriate >>>> place, resulting in excessively long virtual machine startup times. >>>> Call trace: >>>> __alloc_pages_slowpath >>>> if (compact_result == COMPACT_SKIPPED || >>>> compact_result == COMPACT_DEFERRED) >>>> goto nopage; // should exit __alloc_pages_slowpath() from here >>>> >>>> Other unmovable alloctions, like dma_buf, which can be large in a >>>> Linux system, are also unable to allocate memory from CMA, and these >>>> allocations suffer from the same problems described above. In order >>>> to quickly fall back to remote node, we should remove ALLOC_CMA both >>>> in __compaction_suitable() and __isolate_free_page() for unmovable >>>> alloctions. After this fix, starting a 32GB virtual machine with >>>> device passthrough takes only a few seconds. >>> >>> The symptom is obviously bad, but I don't understand this fix. >>> >>> The reason we do ALLOC_CMA is that, even for unmovable allocations, >>> you can create space in non-CMA space by moving migratable pages over >>> to CMA space. This is not a property we want to lose. But I also don't >>> see how it would interfere with your scenario. >> >> The __alloc_pages_slowpath() function was originally intended to exit at >> place 1, but due to __compaction_suitable() always returning true, it >> results in __alloc_pages_slowpath() exiting at place 2 instead. This >> ultimately leads to a significantly longer execution time for >> __alloc_pages_slowpath(). >> >> Call trace: >> __alloc_pages_slowpath >> if (compact_result == COMPACT_SKIPPED || >> compact_result == COMPACT_DEFERRED) >> goto nopage; // place 1 >> __alloc_pages_direct_reclaim() // Reclaim is very expensive >> __alloc_pages_direct_compact() >> if (gfp_mask & __GFP_NORETRY) >> goto nopage; // place 2 >> >> Every time memory allocation goes through the above slower process, it >> ultimately leads to significantly longer virtual machine startup times. > > I still don't follow. Why do you want the allocation to fail? > pin_user_pages_remote(..., FOLL_LONGTERM, ...) first attemps to allocate THP only on local node, and then fall back to remote NUMA nodes if the local allocation fail. For detail, see alloc_pages_mpol(). static struct page *alloc_pages_mpol() { page = __alloc_frozen_pages_noprof(__GFP_THISNODE,...); // 1, try to allocate THP only on local node if (page || !(gpf & __GFP_DIRECT_RECLAIM)) return page; page = __alloc_frozen_pages_noprof(gfp, order, nid, nodemask);//2, fall back to remote NUMA nodes } > The changelog says this is in order to fall back quickly to other > nodes. But there is a full node walk in get_page_from_freelist() > before the allocator even engages reclaim. There is something missing > from the story still. > > But regardless - surely you can see that we can't make the allocator > generally weaker on large requests just because they happen to be > optional in your specific case? > First, try to allocate THP on the local node as much as possible, and then fall back to a remote node if the local allocation fail. This is the default memory allocation strategy when starting virtual machines. >>> There is the compaction_suitable() check in should_compact_retry(), >>> but that only applies when COMPACT_SKIPPED. IOW, it should only happen >>> when compaction_suitable() just now returned false. IOW, a race >>> condition. Which is why it's also not subject to limited retries. >>> >>> What's the exact condition that traps the allocator inside the loop? >> The should_compact_retry() function was not executed, and the slow here >> was mainly due to the execution of __alloc_pages_direct_reclaim(). > > Ok.