From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1424274AbcFMOTM (ORCPT ); Mon, 13 Jun 2016 10:19:12 -0400 Received: from zimbra13.linbit.com ([212.69.166.240]:53448 "EHLO zimbra13.linbit.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423980AbcFMOP4 (ORCPT ); Mon, 13 Jun 2016 10:15:56 -0400 From: Philipp Reisner To: Jens Axboe , linux-kernel@vger.kernel.org Cc: drbd-dev@lists.linbit.com Subject: [PATCH 08/30] drbd: fix regression: protocol A sometimes synchronous, C sometimes double-latency Date: Mon, 13 Jun 2016 16:08:56 +0200 Message-Id: <1465826958-19398-9-git-send-email-philipp.reisner@linbit.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1465826958-19398-1-git-send-email-philipp.reisner@linbit.com> References: <1465826958-19398-1-git-send-email-philipp.reisner@linbit.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Lars Ellenberg Regression introduced with 8.4.5 drbd: application writes may set-in-sync in protocol != C Overwriting the same block (LBA) while a former version is still "in-flight" to the peer (to be exact: we did not receive the P_BARRIER_ACK for its epoch yet) would wait for the full epoch of that former version to be acknowledged by the peer. In synchronous and quasi-synchronous protocols C and B, this may double the latency on overwrites. With protocol A, which is supposed to be asynchronous and only wait for local completion, it is even worse: it would make overwrites quasi-synchronous, they would be hit by the full RTT, which protocol A was specifically meant to avoid, and possibly the additional time it takes to drain the buffers first. Particularly bad for databases, or anything else that does frequent updates to the same blocks (various file system meta data). No impact if >= rtt passes between updates to the same block. Signed-off-by: Philipp Reisner Signed-off-by: Lars Ellenberg --- drivers/block/drbd/drbd_req.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c index 2255dcf..6dbf1f1 100644 --- a/drivers/block/drbd/drbd_req.c +++ b/drivers/block/drbd/drbd_req.c @@ -977,16 +977,20 @@ static void complete_conflicting_writes(struct drbd_request *req) sector_t sector = req->i.sector; int size = req->i.size; - i = drbd_find_overlap(&device->write_requests, sector, size); - if (!i) - return; - for (;;) { - prepare_to_wait(&device->misc_wait, &wait, TASK_UNINTERRUPTIBLE); - i = drbd_find_overlap(&device->write_requests, sector, size); - if (!i) + drbd_for_each_overlap(i, &device->write_requests, sector, size) { + /* Ignore, if already completed to upper layers. */ + if (i->completed) + continue; + /* Handle the first found overlap. After the schedule + * we have to restart the tree walk. */ break; + } + if (!i) /* if any */ + break; + /* Indicate to wake up device->misc_wait on progress. */ + prepare_to_wait(&device->misc_wait, &wait, TASK_UNINTERRUPTIBLE); i->waiting = true; spin_unlock_irq(&device->resource->req_lock); schedule(); -- 2.7.4