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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20240328163424.2781320-24-dhowells@redhat.com> References: <20240328163424.2781320-24-dhowells@redhat.com> <20240328163424.2781320-1-dhowells@redhat.com> To: Christian Brauner , Matthew Wilcox Cc: dhowells@redhat.com, Jeff Layton , Gao Xiang , Dominique Martinet , Steve French , Marc Dionne , Paulo Alcantara , Shyam Prasad N , Tom Talpey , Eric Van Hensbergen , Ilya Dryomov , netfs@lists.linux.dev, linux-cachefs@redhat.com, linux-afs@lists.infradead.org, linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org, ceph-devel@vger.kernel.org, v9fs@lists.linux.dev, linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Latchesar Ionkov , Christian Schoenebeck Subject: Re: [PATCH 23/26] netfs: Cut over to using new writeback code Precedence: bulk X-Mailing-List: linux-cifs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <877901.1712591597.1@warthog.procyon.org.uk> Content-Transfer-Encoding: quoted-printable Date: Mon, 08 Apr 2024 16:53:17 +0100 Message-ID: <877902.1712591597@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.6 David Howells wrote: > + /* Wait for writeback to complete. The writeback engine owns > + * the info in folio->private and may change it until it > + * removes the WB mark. > + */ > + if (folio_wait_writeback_killable(folio)) { > + ret =3D written ? -EINTR : -ERESTARTSYS; > + goto error_folio_unlock; > + } > + It turns out that this really kills performance with fio with as many jobs= as cpus. It's taking up to around 8x longer to complete a pwrite() on averag= e and perf shows a 30% of the CPU cycles are being spent in contention on th= e i_rwsem. The reason this was added here is that writeback cannot take the folio loc= k in order to clean up folio->private without risking deadlock vs the truncatio= n routines (IIRC). I can mitigate this by skipping the wait if folio->private is not set and = if we're not going to attach anything there (see attached). Note that if writeout is ongoing and there is nothing attached to ->private, then we sh= ould not be engaging write-streaming mode and attaching a new netfs_folio (and = if we did, we'd flush the page and wait for it anyway). The other possibility is if we have a writeback group to set. This only applies to ceph for the moment and is something that will need dealing wit= h if/when ceph is made to use this code. David --- diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c index 1eff9413eb1b..279b296f8014 100644 --- a/fs/netfs/buffered_write.c +++ b/fs/netfs/buffered_write.c @@ -255,7 +255,8 @@ ssize_t netfs_perform_write(struct kiocb *iocb, struct= iov_iter *iter, * the info in folio->private and may change it until it * removes the WB mark. */ - if (folio_wait_writeback_killable(folio)) { + if (folio_get_private(folio) && + folio_wait_writeback_killable(folio)) { ret =3D written ? -EINTR : -ERESTARTSYS; goto error_folio_unlock; }