From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4CE31D39418 for ; Thu, 2 Apr 2026 12:49:53 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id B7C696B008C; Thu, 2 Apr 2026 08:49:52 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B535F6B0092; Thu, 2 Apr 2026 08:49:52 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A430B6B0093; Thu, 2 Apr 2026 08:49:52 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 9593C6B008C for ; Thu, 2 Apr 2026 08:49:52 -0400 (EDT) Received: from smtpin30.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 3251D1B8623 for ; Thu, 2 Apr 2026 12:49:52 +0000 (UTC) X-FDA: 84613597824.30.ECB14B7 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf14.hostedemail.com (Postfix) with ESMTP id 5764E10000B for ; Thu, 2 Apr 2026 12:49:50 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=bcoN0v1Q; spf=pass (imf14.hostedemail.com: domain of jlayton@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=jlayton@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1775134190; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=jvZiwQUNxhkgoOZKUWQVfd71ZhiwSyPiWxBKGnQXkE4=; b=Ua9O4yCOaLcbXi1DqhjMQ3D2DgCH5AW9QfAEFl2YIMWoYONPQqJ5qUxZj12W0JbezDhis/ UdEnq6aFRXYf1cjHrZnKxwFErdqVCpxWWvwcI8pt65BhJqdVgWrfwQnQeviQJxZfAkgaWL tChICrFm8UdMHZss1cVFT5KQn7X2OMM= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1775134190; a=rsa-sha256; cv=none; b=7ZDrn545Ft6fKm7HdcvyfoK1HFJX8Kwwd9q2uX9Q0x0S5wKT3UPcXTI47BNJAFpTBcqonF TMdmD41vUw/v60szVoMBQu200/hqMlSyUjNzXWDG+0FgAq/Q9UdIpaK2Rob/RBl1MGRi2b oBg7RhIpl5f4Jg9EZzF8uuK/8CNLFoQ= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=bcoN0v1Q; spf=pass (imf14.hostedemail.com: domain of jlayton@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=jlayton@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 6027B43730; Thu, 2 Apr 2026 12:49:49 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B0C2C116C6; Thu, 2 Apr 2026 12:49:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775134189; bh=61yXAIYjKP3fTku3CSIpgYyU9uL4afIYti7gojKiY+o=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=bcoN0v1QtmRJACQVelMuhNeYBDUZZTUvReGFUf/wg4VN67tQH3U9j/402yH+E1uJS aIqf+8asiSegoCERNov3KwgitwQ8OqGAWCs1TNxIzjRES9bfOt95pgFizKIMb1YiXW hnPuV5Kzk+FdI9Zij1RbKgvuaSn5U9ONVwRfzFzkYr9Q584s+04t6oA+Gu15FSEzzW zFBR9/Gdaa+saxNj0dSpaOaPRo4206ddiaXUth+4S7Eaz5pMoz6cxBEZCYQ+ZbAviF lS6ROKxXjB8NKrQxwXrJ4IIoRaJXsKk/JoTwXnIDvTqUZFNMdKnxdMRQMc3f4NhVv5 yVs/+dINxZ78Q== Message-ID: <629f21c6591903512eb2f3f3c4d6b14a9ac7b91a.camel@kernel.org> Subject: Re: [PATCH 2/4] mm: add atomic flush guard for IOCB_DONTCACHE writeback From: Jeff Layton To: Christoph Hellwig Cc: Alexander Viro , Christian Brauner , Jan Kara , "Matthew Wilcox (Oracle)" , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Mike Snitzer , Chuck Lever , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Thu, 02 Apr 2026 08:49:45 -0400 In-Reply-To: References: <20260401-dontcache-v1-0-1f5746fab47a@kernel.org> <20260401-dontcache-v1-2-1f5746fab47a@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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 YrV6m17b q7mxgmOYpcGBrR4LPUq7vE9Fov5/sfcIkhLvq/LxpujDoRaRtg3yeifVA7AFNrOupq1AcJNnNQinSEknS2rFhffvTFhlivdDa3iltEmufGPxiKPsUTpRncx0Juwblr6r5MOeQRK4pIjCRF6xHZzmp08H+f9+uGB7oqZXGHM6KFRhRx/iax4fDKdN4vftjbPAbFAZUaGNXtiyvB/Nz0Agff3iaQxSWr9tQEta/PQn7qkdJg+UJRFysOroVZbfc/y6g4xvdxsPo6niLBb185NXUswRKyGsLDf5lX9kK Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2026-04-01 at 22:27 -0700, Christoph Hellwig wrote: > On Wed, Apr 01, 2026 at 03:10:59PM -0400, Jeff Layton wrote: > > When the PAGECACHE_TAG_WRITEBACK tag clears after a round of writeback > > completes, all concurrent IOCB_DONTCACHE writers see the tag clear > > simultaneously and submit proportional flushes at once =E2=80=94 a thun= dering > > herd that causes p99.9 tail latency spikes. > >=20 > > Add an AS_DONTCACHE_FLUSHING flag to the address_space and use > > test_and_set_bit() to ensure at most one IOCB_DONTCACHE writer > > flushes at a time. Other writers that find the bit set skip their > > flush entirely. The bit is cleared when the flush completes. >=20 > This sounds like a bad reimplementation of the single writeback thread > :) >=20 > Have you considered stopping to do in-caller writeback for > IOCB_DONTCACHE vs just leaving it to the writeback daeon? >=20 > Either by totally disabling the writeback and just leaving the > dropbehind bit, or by queuing up wb_writeback_work instances for > the ranges, or by just increasing the pressure for the writeback > daemon. Note that with all schemes including the one in this patch > we might eventually run into writeback scalability limits, which > will require multiple writeback workers. I did test a "dropbehind" mode that just set the dropbehind bit without doing the flush at the end of the write. It was better than stock dontcache but the tail latencies were still pretty bad. I think having each writer do some writeback submission work makes a lot of sense. It helps keep the dirty pages below the dirty thresholds and doesn't seem to tax each writing task _too_ much. The trick is avoiding lock contention while doing it. I think what would be ideal would be to have some (lockless) mechanism to say "there is enough data touched by the range just written to kick off a write that's a suitable size for the backing store". Each writer could check that and then kick off writeback for an approprite range. I think this even could be beneficial in the normal buffered write codepath too. Anyway, I'll play around with this idea some more and come back with a v2. Thanks for the review! --=20 Jeff Layton