From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.12]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2BD331EE7DC; Wed, 24 Sep 2025 01:08:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.12 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758676120; cv=none; b=fd8D3Hngw9Mb1bvFvcVAHSQ4Q/d5a+GQqm4IJ8furNM/0cbfUkgu0qgxIPdmzBKA90ABF/8CRDPXOvHJPWb7CRar4nEFCLKozjej0u1zXFUuNs41Oob1R6x8wH02wQW42rRC2qVR3A2w9N4jvObWCiJZ1BlhOvYHNHVwsmdwf5I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758676120; c=relaxed/simple; bh=NMr8YHsAc1qYXXZdTcENL3OIhnYEafGh+AtSGRf6TLc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dgTLpOi6M5qaMY0dbDScLhkQnxIVUvyJn2wODC7raqXfwTwURv+ejB7IwLfyBaqbSGMAy5FfS9rAhKU6xGh7snAxkAlYkQh0koFeVb7euNKtwrYfDJ9ufbg8rMA4lS8sIot3bJInIOAE4ZIHo1J9WmNl9h7a5KSPSjxSvbcKZ4I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com; spf=pass smtp.mailfrom=linux.intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=VONOA13D; arc=none smtp.client-ip=198.175.65.12 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="VONOA13D" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1758676118; x=1790212118; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=NMr8YHsAc1qYXXZdTcENL3OIhnYEafGh+AtSGRf6TLc=; b=VONOA13DVQK8LRrcJpWI69WgbqYFcXN0aHxQ0ZW91dIjxBzWBc9cW64n j7WGH53MAqXmwNnYbf4H3gRJFSLCx//E3FSD8+3evY3Qzh6a5eMDXKWa3 1XH0u04KR0sLuwm6Xex9NknfR0yMn+M3r7b0tQKE/s/zVgyw+iAt2BhK+ oti/psWpXB2NOrRGyRuvQ8dLoHYJgpnKqfvlPbNnTYQBwvn2VueZx/Q0Q rD5Z7DPE/WK5uo5zG7xqY+o8X+N3firvwxUIJyB09Jnp5We1VR4mh687N k8D41rYE967HFXDZpLLUFoo2O/ti0vLrr4fnrw9sF8mkCDi6uEmyeoJLL Q==; X-CSE-ConnectionGUID: 4gXmTAcnSEWR+wo4jpYy8A== X-CSE-MsgGUID: cg5Yij6FTiybfhAxGs1Dhw== X-IronPort-AV: E=McAfee;i="6800,10657,11561"; a="72391785" X-IronPort-AV: E=Sophos;i="6.18,289,1751266800"; d="scan'208";a="72391785" Received: from orviesa001.jf.intel.com ([10.64.159.141]) by orvoesa104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Sep 2025 18:08:37 -0700 X-CSE-ConnectionGUID: fjBTYTLDSiCoIsBzqV7wMQ== X-CSE-MsgGUID: IyKXHQQ5TJ+72htfqHKDig== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.18,289,1751266800"; d="scan'208";a="214035413" Received: from alc-skl-a23.sh.intel.com (HELO [10.239.53.6]) ([10.239.53.6]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Sep 2025 18:08:35 -0700 Message-ID: Date: Wed, 24 Sep 2025 08:27:39 +0800 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/readahead: Skip fully overlapped range To: Jan Kara Cc: Andrew Morton , Matthew Wilcox , Nanhai Zou , Gang Deng , Tianyou Li , Vinicius Gomes , Tim Chen , Chen Yu , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Roman Gushchin References: <20250923035946.2560876-1-aubrey.li@linux.intel.com> <20250922204921.898740570c9a595c75814753@linux-foundation.org> <93f7e2ad-563b-4db5-bab6-4ce2e994dbae@linux.intel.com> Content-Language: en-US From: Aubrey Li In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/23/25 17:57, Jan Kara wrote: > On Tue 23-09-25 13:11:37, Aubrey Li wrote: >> On 9/23/25 11:49, Andrew Morton wrote: >>> On Tue, 23 Sep 2025 11:59:46 +0800 Aubrey Li wrote: >>> >>>> RocksDB sequential read benchmark under high concurrency shows severe >>>> lock contention. Multiple threads may issue readahead on the same file >>>> simultaneously, which leads to heavy contention on the xas spinlock in >>>> filemap_add_folio(). Perf profiling indicates 30%~60% of CPU time spent >>>> there. >>>> >>>> To mitigate this issue, a readahead request will be skipped if its >>>> range is fully covered by an ongoing readahead. This avoids redundant >>>> work and significantly reduces lock contention. In one-second sampling, >>>> contention on xas spinlock dropped from 138,314 times to 2,144 times, >>>> resulting in a large performance improvement in the benchmark. >>>> >>>> w/o patch w/ patch >>>> RocksDB-readseq (ops/sec) >>>> (32-threads) 1.2M 2.4M >>> >>> On which kernel version? In recent times we've made a few readahead >>> changes to address issues with high concurrency and a quick retest on >>> mm.git's current mm-stable branch would be interesting please. >> >> I'm on v6.16.7. Thanks Andrew for the information, let me check with mm.git. > > I don't expect much of a change for this load but getting test result with > mm.git as a confirmation would be nice. Yes, the hotspot remains on mm.git:mm-stable branch. - 88.68% clone3 - 88.68% start_thread - 88.68% reader_thread - 88.27% syscall entry_SYSCALL_64_after_hwframe do_syscall_64 ksys_readahead generic_fadvise force_page_cache_ra page_cache_ra_unbounded filemap_add_folio __filemap_add_folio _raw_spin_lock_irq - do_raw_spin_lock native_queued_spin_lock_slowpath > Also, based on the fact that the > patch you propose helps, this looks like there are many threads sharing one > struct file which race to read the same content. That is actually rather > problematic for current readahead code because there's *no synchronization* > on updating file's readhead state. So threads can race and corrupt the > state in interesting ways under one another's hands. On rare occasions I've > observed this with heavy NFS workload where the NFS server is > multithreaded. Since the practical outcome is "just" reduced read > throughput / reading too much, it was never high enough on my priority list > to fix properly (I do have some preliminary patch for that laying around > but there are some open questions that require deeper thinking - like how > to handle a situation where one threads does readahead, filesystem requests > some alignment of the request size after the fact, so we'd like to update > readahead state but another thread has modified the shared readahead state > in the mean time). But if we're going to work on improving behavior of > readahead for multiple threads sharing readahead state, fixing the code so > that readahead state is at least consistent is IMO the first necessary > step. And then we can pile more complex logic on top of that. This makes sense. I actually had a version using atomic operations to update ra in my patch, but I found that ra is also updated in other paths without synchronization, so I dropped the atomic operations before sending the patch. Let me check what I can do for this. Have you put your preliminary patch somewhere? Thanks, -Aubrey > > Honza