From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752824AbaJ1UBA (ORCPT ); Tue, 28 Oct 2014 16:01:00 -0400 Received: from mail-qa0-f51.google.com ([209.85.216.51]:59685 "EHLO mail-qa0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751153AbaJ1UA7 (ORCPT ); Tue, 28 Oct 2014 16:00:59 -0400 Date: Tue, 28 Oct 2014 16:00:55 -0400 From: Tejun Heo To: Christian Borntraeger Cc: Kent Overstreet , Jens Axboe , Christoph Hellwig , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , linux-s390 Subject: Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance) Message-ID: <20141028200055.GA8205@htj.dyndns.org> References: <544FF00B.8050403@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <544FF00B.8050403@de.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Tue, Oct 28, 2014 at 08:35:39PM +0100, Christian Borntraeger wrote: > when going from 3.17 to 3.18-rc2 cpu hotplug become horrible slow on some KVM guests on s390 > > I was able to bisect this to > > commit 9eca80461a45177e456219a9cd944c27675d6512 > ("Revert "blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe") That removes the earlier kludge to avoid the RCU delay so RCU latencies are expected to show up right after; however, the following patches implement proper fix for the problem and the latencies shouldn't be visible afterwards. So, 17497acbdce9 ("blk-mq, percpu_ref: start q->mq_usage_counter in atomic mode") should remove the latencies again. It doesn't? Thanks. -- tejun