From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lai Jiangshan Subject: Re: [PATCH] lglock: add read-preference local-global rwlock Date: Tue, 05 Mar 2013 23:54:40 +0800 Message-ID: <51361540.3060603@cn.fujitsu.com> References: <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <512E7879.20109@linux.vnet.ibm.com> <5130E8E2.50206@cn.fujitsu.com> <20130301182854.GA3631@redhat.com> <20130302170656.GB29769@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130302170656.GB29769@redhat.com> Sender: linux-doc-owner@vger.kernel.org To: Oleg Nesterov Cc: Michel Lespinasse , "Srivatsa S. Bhat" , Lai Jiangshan , linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, namhyung@kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, vincent.guittot@linaro.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org List-Id: linux-arch.vger.kernel.org On 03/03/13 01:06, Oleg Nesterov wrote: > On 03/02, Michel Lespinasse wrote: >> >> My version would be slower if it needs to take the >> slow path in a reentrant way, but I'm not sure it matters either :) > > I'd say, this doesn't matter at all, simply because this can only happen > if we race with the active writer. > It can also happen when interrupted. (still very rarely) arch_spin_trylock() ------->interrupted, __this_cpu_read() returns 0. arch_spin_trylock() fails slowpath, any nested will be slowpath too. ... ..._read_unlock() <-------interrupt __this_cpu_inc() .... I saw get_online_cpu_atomic() is called very frequent. And the above thing happens in one CPU rarely, but how often it happens in the whole system if we have 4096 CPUs? (I worries to much. I tend to remove FALLBACK_BASE now, we should add it only after we proved we needed it, this part is not proved) Thanks, Lai From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([222.73.24.84]:42634 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1751935Ab3CEQ0V (ORCPT ); Tue, 5 Mar 2013 11:26:21 -0500 Message-ID: <51361540.3060603@cn.fujitsu.com> Date: Tue, 05 Mar 2013 23:54:40 +0800 From: Lai Jiangshan MIME-Version: 1.0 Subject: Re: [PATCH] lglock: add read-preference local-global rwlock References: <512C7A38.8060906@linux.vnet.ibm.com> <512CC509.1050000@linux.vnet.ibm.com> <512D0D67.9010609@linux.vnet.ibm.com> <512E7879.20109@linux.vnet.ibm.com> <5130E8E2.50206@cn.fujitsu.com> <20130301182854.GA3631@redhat.com> <20130302170656.GB29769@redhat.com> In-Reply-To: <20130302170656.GB29769@redhat.com> Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=UTF-8 Sender: linux-arch-owner@vger.kernel.org List-ID: To: Oleg Nesterov Cc: Michel Lespinasse , "Srivatsa S. Bhat" , Lai Jiangshan , linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, namhyung@kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, vincent.guittot@linaro.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Message-ID: <20130305155440.6M3KFncCXZtHW1NpYwLJGhU69ot0dN0VUqHs9QExz78@z> On 03/03/13 01:06, Oleg Nesterov wrote: > On 03/02, Michel Lespinasse wrote: >> >> My version would be slower if it needs to take the >> slow path in a reentrant way, but I'm not sure it matters either :) > > I'd say, this doesn't matter at all, simply because this can only happen > if we race with the active writer. > It can also happen when interrupted. (still very rarely) arch_spin_trylock() ------->interrupted, __this_cpu_read() returns 0. arch_spin_trylock() fails slowpath, any nested will be slowpath too. ... ..._read_unlock() <-------interrupt __this_cpu_inc() .... I saw get_online_cpu_atomic() is called very frequent. And the above thing happens in one CPU rarely, but how often it happens in the whole system if we have 4096 CPUs? (I worries to much. I tend to remove FALLBACK_BASE now, we should add it only after we proved we needed it, this part is not proved) Thanks, Lai