From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from m16.mail.126.com (m16.mail.126.com [117.135.210.7]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0D3F32F3B; Wed, 18 Dec 2024 02:15:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=117.135.210.7 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734488146; cv=none; b=N9uby+wgKxGq3to33t2D/vwOvaSka9v8832H1vZPtyK+nXtiZ2nDibDeJTP30LUnDQ3dJ3dDlu1UtldGZg0On9bTdfyl18YFWRhVknt5MCPz5P1a3ZV8B17qIQf2+e+y+42XZKY8Wt8dZGd+aMxV8gfAMXiFq5aKW8PXA1H03Iw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734488146; c=relaxed/simple; bh=PGcVUbTDxYfjAwzNSqwhREh3pDsx5gmJswCYMRavpjI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lMsWDoitIy8CDw7sgPbmbnSqvfzVKevFMYE8qpepkCKkH2jY9sd6GXnsUd9+XwkweWIKiaPvvwJSkQGVCAVIiOwZRQ4A3SMzuUkWNqAFSL1qFSy8wEDm6T9RwKn6t4rnOPB6KZ/1qk9WNSHXVGnw6LIQjFT+9whRdYynsJYW7FU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com; spf=pass smtp.mailfrom=126.com; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b=AImudZie; arc=none smtp.client-ip=117.135.210.7 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=126.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=126.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=126.com header.i=@126.com header.b="AImudZie" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=126.com; s=s110527; h=Message-ID:Date:MIME-Version:Subject:From: Content-Type; bh=53Hsg8krc/9Eb72fAtYb1b6FfRkOMXJEiWWXPojXaYI=; b=AImudZie8AJIhZ1ru//aQrHQtuopGbEdVrjvBBH+rsob0m0p9EaHck8PwKz5gc L929EGqvAgeR04MpKCTQqwro5JtX5/3xZRyuJoT8xmDhI96HAEom3/Gz7yghvkOf kS3SofgDfXmSnqppdhnL8tCq+qfFZ0pkEiNCmT1bBN+Hk= Received: from [172.21.22.210] (unknown []) by gzga-smtp-mtada-g1-1 (Coremail) with SMTP id _____wD3l5IqMGJneqnIAQ--.29653S2; Wed, 18 Dec 2024 10:15:07 +0800 (CST) Message-ID: <93cf1aee-70df-426f-a3c0-1db8068bd59a@126.com> Date: Wed, 18 Dec 2024 10:15:06 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> From: Ge Yang In-Reply-To: <20241217155551.GA37530@cmpxchg.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:_____wD3l5IqMGJneqnIAQ--.29653S2 X-Coremail-Antispam: 1Uf129KBjvJXoWxXF4xCw17CF4fGrWxCr48JFb_yoWrGry8pF W8ZrnFkws8ZFWDKrn2yw1v9a4jga18tF4UJw1qvrykursIkF9IkF1DtFyUCFyUXr15tayS qFW8u3sxAa15Za7anT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07UqYLPUUUUU= X-CM-SenderInfo: 51dqwwjhrrila6rslhhfrp/1tbiOhq4G2dhmi52-wACs6 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. > > 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().