From patchwork Wed Sep 26 21:08:47 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Josef Bacik X-Patchwork-Id: 10616887 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 802A015E8 for ; Wed, 26 Sep 2018 21:10:10 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 719062B7F2 for ; Wed, 26 Sep 2018 21:10:10 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 661C42B7FC; Wed, 26 Sep 2018 21:10:10 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0B7102B7F2 for ; Wed, 26 Sep 2018 21:10:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727018AbeI0DXu (ORCPT ); Wed, 26 Sep 2018 23:23:50 -0400 Received: from mail-qt1-f195.google.com ([209.85.160.195]:32943 "EHLO mail-qt1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726727AbeI0DXu (ORCPT ); Wed, 26 Sep 2018 23:23:50 -0400 Received: by mail-qt1-f195.google.com with SMTP id i10-v6so536935qtp.0 for ; Wed, 26 Sep 2018 14:09:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=toxicpanda-com.20150623.gappssmtp.com; s=20150623; h=from:to:subject:date:message-id; bh=3SZMKRYMVmDdRPjAvl+mIDUi0cvHC1rXOW2VG9ITyAY=; b=cf0UqgF5vjizMzXoCoy5s9HUKhdGw0IoCF3VyKpHVIjPWKPFgjoc1HJC7VLl39IoKl 2BPgi6+mlIfE1dZSZ4hDJFTW2kWTFI/vj9bly3PC3kXOU+oOkNjEvv5h57YcKixZ5dFP ZKioL3FKLxdFbmsNT964T+bKw0MTjj0qEnP71rvruambf7dCedyENzfjmr3CAaYCocan 0KKg57epUNSVvVmSZ9XPHhqjeJuyScPsDrbe0xvYDKdrspP4WeLb5cwIJ9jfTEUGpugP O2ADcCrG3RumxPRoQfJTIvoufAC2rkU5qzOQR4921VmXJkNnAD24e4PshryVUqKW3Gtq Ztmg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:subject:date:message-id; bh=3SZMKRYMVmDdRPjAvl+mIDUi0cvHC1rXOW2VG9ITyAY=; b=bmjLrasaC/wras5CtnsXzM9T+GqFHzpoMk1Qn/jrenpGtMHQAQbELV4LTmZrkJ8r56 cC3Qfik8Om8kJaTVLctSCL2Oel/KaeMXnHVSlq/tST9OhOlVoRhVem8FJbblreWiBlu9 Y5NOjuJs85sHe/3fyTXtU2k8Yq9jRBFxQxBFRKSeElyQLWSyZfj9egN9str3jps4i18d 3Y1y4a3mKD4n+KqFKV//+flU4DAA7fMG+lTVzPInmBeQHvd+UUi30HUO7K3rsRTkGPLZ A/UEsNOOw8aOGRwfg2rB77ZG4f55U9flnBXcQESDw/J8KusXjmWHqG9r/Ull6eTaFtUA 53yg== X-Gm-Message-State: ABuFfogR+YB3n8g2k78WjCNy9WUY+EMBPY/OtPl4rk5iRBsJkpugvzeK 9osd1NbnODVsfuuB8XDuYEIohQ== X-Google-Smtp-Source: ACcGV62M6+hEbeqHqVou90I4+huN9MbHVcT7hhBEGLDMQSmUWN0TB4fl/YOjB8uW0onQoW2aCQZr5w== X-Received: by 2002:ac8:3097:: with SMTP id v23-v6mr6132402qta.335.1537996139508; Wed, 26 Sep 2018 14:08:59 -0700 (PDT) Received: from localhost ([107.15.81.208]) by smtp.gmail.com with ESMTPSA id h37-v6sm65854qtb.37.2018.09.26.14.08.58 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 26 Sep 2018 14:08:58 -0700 (PDT) From: Josef Bacik To: kernel-team@fb.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, tj@kernel.org, linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org, riel@redhat.com, linux-mm@kvack.org, linux-btrfs@vger.kernel.org Subject: [RFC][PATCH 0/9][V2] drop the mmap_sem when doing IO in the fault path Date: Wed, 26 Sep 2018 17:08:47 -0400 Message-Id: <20180926210856.7895-1-josef@toxicpanda.com> X-Mailer: git-send-email 2.14.3 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP v1->v2: - reworked so it only affects x86, since its the only arch I can build and test. - fixed the fact that do_page_mkwrite wasn't actually sending ALLOW_RETRY down to ->page_mkwrite. - fixed error handling in do_page_mkwrite/callers to explicitly catch VM_FAULT_RETRY. - fixed btrfs to set ->cached_page properly. This time I've verified that the ->page_mkwrite retry path is actually getting used (apparently I only verified the read side last time). xfstests is still running but it passed the couple of mmap tests I ran directly. Again this is an RFC, I'm still doing a bunch of testing, but I'd appreciate comments on the overall strategy. -- Original message -- Now that we have proper isolation in place with cgroups2 we have started going through and fixing the various priority inversions. Most are all gone now, but this one is sort of weird since it's not necessarily a priority inversion that happens within the kernel, but rather because of something userspace does. We have giant applications that we want to protect, and parts of these giant applications do things like watch the system state to determine how healthy the box is for load balancing and such. This involves running 'ps' or other such utilities. These utilities will often walk /proc//whatever, and these files can sometimes need to down_read(&task->mmap_sem). Not usually a big deal, but we noticed when we are stress testing that sometimes our protected application has latency spikes trying to get the mmap_sem for tasks that are in lower priority cgroups. This is because any down_write() on a semaphore essentially turns it into a mutex, so even if we currently have it held for reading, any new readers will not be allowed on to keep from starving the writer. This is fine, except a lower priority task could be stuck doing IO because it has been throttled to the point that its IO is taking much longer than normal. But because a higher priority group depends on this completing it is now stuck behind lower priority work. In order to avoid this particular priority inversion we want to use the existing retry mechanism to stop from holding the mmap_sem at all if we are going to do IO. This already exists in the read case sort of, but needed to be extended for more than just grabbing the page lock. With io.latency we throttle at submit_bio() time, so the readahead stuff can block and even page_cache_read can block, so all these paths need to have the mmap_sem dropped. The other big thing is ->page_mkwrite. btrfs is particularly shitty here because we have to reserve space for the dirty page, which can be a very expensive operation. We use the same retry method as the read path, and simply cache the page and verify the page is still setup properly the next pass through ->page_mkwrite(). I've tested these patches with xfstests and there are no regressions. Let me know what you think. Thanks, Josef