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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH 2/4] writeback: name the bdi in the slow writeback report Message-ID: References: <20261106-writeback_stall_report-v1-0-bf0c3b9023a5@debian.org> <20261106-writeback_stall_report-v1-2-bf0c3b9023a5@debian.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Debian-User: leitao X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam09 X-Stat-Signature: pu5yskdn87r7u7rju59ruxmgtsrmg5a4 X-Rspamd-Queue-Id: 53E644000B X-Rspam-User: X-HE-Tag: 1793980131-342912 X-HE-Meta: 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 Oxt+i1Ih d9HN0Sx/pCB22p/m/bryDCCn5gdrElU7YEIYiX62n61DnWzw5uTlGAGtxO5mqYK4FQfYeMYXWF8s62C4SWLqtEyiTey4Xk4oobE76Qwcu/FqyJIgrUnncVv0WYrGKVWwGzb4WIN5Z7xJifLx65HQykPkyIR3Z0PrhNRSm2x5fxfHdfIMeQnXM9hU4/QR20Hsy8dHRwrdeVssI4D1KD/mhf8GnOkGMocmbUnY2SThUldcTgy75v6fIqh/Eglk6WllbZMeb1r1HPhYq1dvuXzHKAfI1UA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Fri, Nov 06, 2026 at 03:27:49PM +0100, Jan Kara wrote: > On Fri 06-11-26 04:26:32, Breno Leitao wrote: > > The slow writeback report names the waiting task and how long it has > > waited, but not what it is waiting on. On a machine with several > > filesystems there is no way to tell which device is slow. > > > > Pass the bdi to wb_wait_for_completion() and print its name on a second > > line, together with the number of work items still outstanding. A sync > > issues one work item per cgroup wb with dirty inodes, so the count also > > shows whether a single cgroup is holding up the others. > > > > wb_wait_for_completion() took the bdi until commit 5b9cce4c7eb0 > > ("writeback: Generalize and expose wb_completion") made it usable for > > the memcg foreign flushes. memcg tracks those by bdi id only and passes > > NULL, which bdi_dev_name() prints as "(unknown)". > > > > The first line does not change, so anything matching on it keeps > > working. A sync on a throttled null_blk device now reports: > > > > INFO: The task sync:180 has been waiting for writeback completion for more than 28 seconds. > > bdi 252:0, work items left: 1 > > > > Signed-off-by: Breno Leitao > > Dunno, I'd put everything on one line. Otherwise things can get interleaved > by other messages. Kernel messages are not really an ABI, we can change > them... Ack! I will get them in a single line and respin, --breno