From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:41640 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726529AbeIKNSq (ORCPT ); Tue, 11 Sep 2018 09:18:46 -0400 Date: Tue, 11 Sep 2018 16:20:24 +0800 From: Ming Lei To: "jianchao.wang" Cc: linux-kernel@vger.kernel.org, Tejun Heo , Kent Overstreet , linux-block@vger.kernel.org Subject: Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit() Message-ID: <20180911082023.GA5360@ming.t460p> References: <20180909125824.9150-1-ming.lei@redhat.com> <20180910161135.GA27430@ming.t460p> <20180911040323.GC30977@ming.t460p> <771fa1eb-85d3-2ff2-6070-207874da85b6@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <771fa1eb-85d3-2ff2-6070-207874da85b6@oracle.com> Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Tue, Sep 11, 2018 at 12:40:36PM +0800, jianchao.wang wrote: > Hi Ming > > On 09/11/2018 12:03 PM, Ming Lei wrote: > > After the refcount is switched to atomic mode, the whole counting is > > done on the atomic part. Then if the refcount need to switch to percpu mode > > again, all percpu part of the counter is re-initialized as zero simply. This > > is invariant with/without this patch. > > Does the "whole counting" here means ? > > (long)(sum (every cpu's refcounter)) + atomic refcounter No. We only check the 'whole counting' at atomic mode, so it is the atomic part('ref->count'), please see percpu_ref_put_many(), in which ref->release() is only called at atomic mode. At percpu mode, the ref-count is only decreased/increased on the local CPU. > > and when switch to atomic mode, there could be value left in atomic refcounter. > then the unsigned long percpu refcounter cound be decreased from 0. > > From another angle, one request could be ended on a different cpu from the one where > it is issued. Right. But, In the fast path(either completion or issue path), the .q_usage_counter is only increased/decreased, which can be done in either atomic or percpu mode. And when we want to check the queued/inflight requests, the .q_usage_counter has to be killed first by switching to atomic mode, such as in all kinds of uses of blk_freeze_queue_start(). Thanks, Ming