From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757266Ab2IDOXq (ORCPT ); Tue, 4 Sep 2012 10:23:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]:15460 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757226Ab2IDOXo (ORCPT ); Tue, 4 Sep 2012 10:23:44 -0400 Date: Tue, 4 Sep 2012 10:23:39 -0400 From: Vivek Goyal To: Tao Ma Cc: linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe Subject: Re: [PATCH V2] block/throttle: Add IO throttled information in blkio.throttle. Message-ID: <20120904142339.GE13768@redhat.com> References: <1346390109-3169-1-git-send-email-tm@tao.ma> <20120904133528.GB13768@redhat.com> <50460C61.6050705@tao.ma> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50460C61.6050705@tao.ma> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 04, 2012 at 10:12:49PM +0800, Tao Ma wrote: > On 09/04/2012 09:35 PM, Vivek Goyal wrote: > > On Fri, Aug 31, 2012 at 01:15:09PM +0800, Tao Ma wrote: > > > > [..] > >> diff --git a/block/blk-throttle.c b/block/blk-throttle.c > >> index 1588c2d..9317d71 100644 > >> --- a/block/blk-throttle.c > >> +++ b/block/blk-throttle.c > >> @@ -46,6 +46,8 @@ struct tg_stats_cpu { > >> struct blkg_rwstat service_bytes; > >> /* total IOs serviced, post merge */ > >> struct blkg_rwstat serviced; > >> + /* total IOs queued, not submitted to the underlying device. */ > >> + struct blkg_rwstat io_queued; > >> }; > > > > Couple of questions. > > > > - blkg_rwstat is "unsigned" and io_queued can go negative too (Because > > throttled bio can very well be dispatched from other cpu from a worker > > thread). So is it a good idea to represent a negative number with > > unsingned type? > > > > - As this stat is per cpu, a reader might very well see negative (or a > > huge unsigned value) as number of io_queued. Not sure if that is acceptable. > > How would user space come to know whether it is a valid value or not. I > > thought per cpu stats are good for continuously increasing values but > > not necessarily for values which can increase as well as decrease. > You are right. So I should just use throtl_grp->nr_queued to display the > total numbers of ios being throttled and I guess a rcu_read_lock should > be enough for me to access that data. Not sure how rcu_read_lock() is going to help. Can you explain a bit more. For 64bit, we should not require any locking as updation always happens under queue_lock(for io_queued). And lockless reading should be just fine as updates to 64bit values will be atomic. Only problem is reading of 64bit io_queued on 32bit platforms. As updation always happens under queue_lock, we don't gain anything by making this stat per cpu. And for 32bit, we probably can updation/reading using sequence counter to make sure we don't get invalid values and read will still be lockless. Thanks Vivek