From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes Date: Tue, 22 Nov 2011 21:02:31 +0800 Message-ID: <20111122130231.GA18638@localhost> References: <20111121130342.211953629@intel.com> <20111121131215.905222115@intel.com> <20111122001127.GG4017@quack.suse.cz> <20111122092110.GB12864@localhost> <20111122122157.GB8058@quack.suse.cz> <20111122123001.GA16054@localhost> <20111122124811.GD8058@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "linux-fsdevel@vger.kernel.org" , Peter Zijlstra , Christoph Hellwig , Andrew Morton , LKML To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20111122124811.GD8058@quack.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Tue, Nov 22, 2011 at 08:48:11PM +0800, Jan Kara wrote: > On Tue 22-11-11 20:30:01, Wu Fengguang wrote: > > > > @@ -1743,6 +1738,8 @@ void account_page_dirtied(struct page *p > > > > __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED); > > > > task_dirty_inc(current); > > > > task_io_account_write(PAGE_CACHE_SIZE); > > > > + current->nr_dirtied++; > > > > + __get_cpu_var(bdp_ratelimits)++; > > > I think you need preempt_disable() and preempt_enable() pair around > > > __get_cpu_var(). Otherwise a process could get rescheduled in the middle of > > > read-modify-write cycle... > > > > Hmm, I'm not worried about it at all, because bdp_ratelimits don't > > need to be accurate. In normal cases it won't even trigger one single > > call to balance_dirty_pages(). > I agree regarding the accuracy. But the CPU can change when the process > is scheduled again. So you could modify counter of a CPU you are not > running on. And that can cause bad things... Will modifying another CPU's per-cpu data lead to more serious problems than inaccuracy? If not, it would be fine. bdp_ratelimits is only meant to be a coarse grained safeguard after all :-) > > btw, account_page_dirtied() is called inside spinlock, will it be > > sufficient? > Currently it is not enough in real-time kernels and when sleeping > spinlocks work gets merged it won't be enough even in standard kernels... > And in kernels where spinlock means preemption is disabled > preempt_enable/disable will be almost for free... I see, spinlock won't be a general superset of preempt_enable/disable indeed. Thanks, Fengguang