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[2604:1380:45d1:ec00::1]) by mx.google.com with ESMTPS id jr11-20020a05622a800b00b004237f85a05esi1413878qtb.540.2023.11.30.09.50.20 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Nov 2023 09:50:20 -0800 (PST) Received-SPF: pass (google.com: domain of linux-nfs+bounces-226-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=k20201202 header.b=m5uyhFHH; spf=pass (google.com: domain of linux-nfs+bounces-226-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) smtp.mailfrom="linux-nfs+bounces-226-linux.lists.archive=gmail.com@vger.kernel.org"; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: from smtp.subspace.kernel.org (wormhole.subspace.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ny.mirrors.kernel.org (Postfix) with ESMTPS id 283C51C20F03 for ; Thu, 30 Nov 2023 17:50:20 +0000 (UTC) Received: from localhost.localdomain (localhost.localdomain [127.0.0.1]) by smtp.subspace.kernel.org (Postfix) with ESMTP id A3BE2584D0; Thu, 30 Nov 2023 17:50:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="m5uyhFHH" X-Original-To: linux-nfs@vger.kernel.org Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 8342A584C9; Thu, 30 Nov 2023 17:50:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 40497C433C7; Thu, 30 Nov 2023 17:50:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1701366616; bh=k2xu/AFCVYNd82jmlhXyCXssmQpyJnshGT7Qg/1LpKk=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=m5uyhFHHg6LWMqfxLfxZmF/CM6MFDQNT6gndFI+5KU2bL59X3uoZMjzJcFkJRQJ7O AF+FipoXB+nQf6D5/1qZXWrKPoWbPQD+R/Gq9ukMbhkfTDfuR0hEEC2nSCqq9bJUBs KRv09H+yJJUVjSbB7dxo91/SJ5IGorHvoBGASI+8PTXx6qJ0ybHzbzQpaevkzLGpzL fRm/BFh4i9CFmOgD66LJ/0mfgXZCz/UVO8KpRMxohZ/qJBTr4encQp6NcSpU0vRIlh 2XkfCAb6wBnI+nKC5pLCtXazOPDaqVvlvKz+f7sGlXcMIoxCJSuQSO+EPctRTrTu2N o6tQw2wI6O/Nw== Message-ID: <68b6743f8c095177f5c99876627861f0fbf48edc.camel@kernel.org> Subject: Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work. From: Jeff Layton To: Chuck Lever , NeilBrown Cc: Al Viro , Christian Brauner , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org Date: Thu, 30 Nov 2023 12:50:14 -0500 In-Reply-To: References: <170112272125.7109.6245462722883333440@noble.neil.brown.name> <170113056683.7109.13851405274459689039@noble.neil.brown.name> <170114025065.7109.15330780753462853254@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; 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QiErRVPpB7VzA/pYV5Mdy7GMbPjmO0IpoL0tVZ8JvUzUZXB3ErS/dJflvboAAQeLpLCkQjqZiQ/DCmgJCrBJst9Xc7YsKKS379Tc3GU33HNSpaOxs2NwfzoesyjKU+P35czvXWTtj7KVVSj3SgzzFk+gLx8y2Nvt9iESdZ1Ustv8tipDsGcvIZ43MQwqU9YbLg8k4V9ch+Mo8SE+C0jyZYDCE2ZGf3OztvtSYMsTnF6/luzVyej1AFVYjKHORzNoTwdHUeC+9/07GO0bMYTPXYvJ/vxBFm3oniXyhgb5FtABEBAAGJAh8EGAECAAkFAk6V0TwCGwwACgkQAA5oQRlWghXhZRAAyycZ2DDyXh2bMYvI8uHgCbeXfL3QCvcw2XoZTH2l2umPiTzrCsDJhgwZfG9BDyOHaYhPasd5qgrUBtjjUiNKjVM+Cx1DnieR0dZWafnqGv682avPblfi70XXr2juRE/fSZoZkyZhm+nsLuIcXTnzY4D572JGrpRMTpNpGmitBdh1l/9O7Fb64uLOtA5Qj5jcHHOjL0DZpjmFWYKlSAHmURHrE8M0qRryQXvlhoQxlJR4nvQrjOPMsqWD5F9mcRyowOzr8amasLv43w92rD2nHoBK6rbFE/qC7AAjABEsZq8+TQmueN0maIXUQu7TBzejsEbV0i29z+kkrjU2NmK5pcxgAtehVxpZJ14LqmN6E0suTtzjNT1eMoqOPrMSx+6vOCIuvJ/MVYnQgHhjtPPnU86mebTY5Loy9YfJAC2EVpxtcCbx2KiwErTndEyWL+GL53LuScUD7tW8vYbGIp4RlnUgPLbqpgssq2gwYO9m75FGuKuB2+2bCGajqalid5nzeq9v7cYLLRgArJfOIBWZrHy2m0C+pFu9DSuV6SNr2dvMQUv1V58h0FaSOxHVQnJdnoHn13g/CKKvyg2EMrMt/EfcXgvDwQbnG9we4xJiWOIOcsvrWcB6C6lWBDA+In7w7SXnnok kZWuOsJdJQdmwlWC5L5ln9xgfr/4mOY38B0U= Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.1 (3.50.1-1.fc39) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2023-11-28 at 10:34 -0500, Chuck Lever wrote: > On Tue, Nov 28, 2023 at 01:57:30PM +1100, NeilBrown wrote: > >=20 > > (trimmed cc...) > >=20 > > On Tue, 28 Nov 2023, Chuck Lever wrote: > > > On Tue, Nov 28, 2023 at 11:16:06AM +1100, NeilBrown wrote: > > > > On Tue, 28 Nov 2023, Chuck Lever wrote: > > > > > On Tue, Nov 28, 2023 at 09:05:21AM +1100, NeilBrown wrote: > > > > > >=20 > > > > > > I have evidence from a customer site of 256 nfsd threads adding= files to > > > > > > delayed_fput_lists nearly twice as fast they are retired by a s= ingle > > > > > > work-queue thread running delayed_fput(). As you might imagine= this > > > > > > does not end well (20 million files in the queue at the time a = snapshot > > > > > > was taken for analysis). > > > > > >=20 > > > > > > While this might point to a problem with the filesystem not han= dling the > > > > > > final close efficiently, such problems should only hurt through= put, not > > > > > > lead to memory exhaustion. > > > > >=20 > > > > > I have this patch queued for v6.8: > > > > >=20 > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/com= mit/?h=3Dnfsd-next&id=3Dc42661ffa58acfeaf73b932dec1e6f04ce8a98c0 > > > > >=20 > > > >=20 > > > > Thanks.... > > > > I think that change is good, but I don't think it addresses the pro= blem > > > > mentioned in the description, and it is not directly relevant to th= e > > > > problem I saw ... though it is complicated. > > > >=20 > > > > The problem "workqueue ... hogged cpu..." probably means that > > > > nfsd_file_dispose_list() needs a cond_resched() call in the loop. > > > > That will stop it from hogging the CPU whether it is tied to one CP= U or > > > > free to roam. > > > >=20 > > > > Also that work is calling filp_close() which primarily calls > > > > filp_flush(). > > > > It also calls fput() but that does minimal work. If there is much = work > > > > to do then that is offloaded to another work-item. *That* is the > > > > workitem that I had problems with. > > > >=20 > > > > The problem I saw was with an older kernel which didn't have the nf= sd > > > > file cache and so probably is calling filp_close more often. > > >=20 > > > Without the file cache, the filp_close() should be handled directly > > > by the nfsd thread handling the RPC, IIRC. > >=20 > > Yes - but __fput() is handled by a workqueue. > >=20 > > >=20 > > >=20 > > > > So maybe > > > > my patch isn't so important now. Particularly as nfsd now isn't cl= osing > > > > most files in-task but instead offloads that to another task. So t= he > > > > final fput will not be handled by the nfsd task either. > > > >=20 > > > > But I think there is room for improvement. Gathering lots of files > > > > together into a list and closing them sequentially is not going to = be as > > > > efficient as closing them in parallel. > > >=20 > > > I believe the file cache passes the filps to the work queue one at > >=20 > > nfsd_file_close_inode() does. nfsd_file_gc() and nfsd_file_lru_scan() > > can pass multiple. > >=20 > > > a time, but I don't think there's anything that forces the work > > > queue to handle each flush/close completely before proceeding to the > > > next. > >=20 > > Parallelism with workqueues is controlled by the work items (struct > > work_struct). Two different work items can run in parallel. But any > > given work item can never run parallel to itself. > >=20 > > The only work items queued on nfsd_filecache_wq are from > > nn->fcache_disposal->work. > > There is one of these for each network namespace. So in any given > > network namespace, all work on nfsd_filecache_wq is fully serialised. >=20 > OIC, it's that specific case you are concerned with. The per- > namespace laundrette was added by: >=20 > 9542e6a643fc ("nfsd: Containerise filecache laundrette") >=20 > It's purpose was to confine the close backlog to each container. >=20 > Seems like it would be better if there was a struct work_struct > in each struct nfsd_file. That wouldn't add real backpressure to > nfsd threads, but it would enable file closes to run in parallel. >=20 I like this idea. That seems a lot simpler than all of this weirdo queueing of delayed closes that we do. --=20 Jeff Layton