From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755647Ab0ANC6A (ORCPT ); Wed, 13 Jan 2010 21:58:00 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755248Ab0ANC57 (ORCPT ); Wed, 13 Jan 2010 21:57:59 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:54965 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752646Ab0ANC56 (ORCPT ); Wed, 13 Jan 2010 21:57:58 -0500 Message-ID: <4B4E87C8.7080402@cn.fujitsu.com> Date: Thu, 14 Jan 2010 10:56:08 +0800 From: Lai Jiangshan User-Agent: Thunderbird 2.0.0.6 (Windows/20070728) MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com, Mathieu Desnoyers CC: Steven Rostedt , Oleg Nesterov , Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, dipankar@in.ibm.com Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier References: <20100110014456.GG25790@Krystal> <1263089578.2231.22.camel@frodo> <20100110052508.GG9044@linux.vnet.ibm.com> <1263124209.28171.3798.camel@gandalf.stny.rr.com> <20100110174512.GH9044@linux.vnet.ibm.com> <20100110182423.GA22821@Krystal> <20100111011705.GJ9044@linux.vnet.ibm.com> <20100111042521.GB32213@Krystal> <20100111162537.GD6632@linux.vnet.ibm.com> <20100111202104.GA2816@Krystal> <20100111214803.GG6632@linux.vnet.ibm.com> In-Reply-To: <20100111214803.GG6632@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Paul E. McKenney wrote: > On Mon, Jan 11, 2010 at 03:21:04PM -0500, Mathieu Desnoyers wrote: >> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: >>> On Sun, Jan 10, 2010 at 11:25:21PM -0500, Mathieu Desnoyers wrote: >>>> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: >>>> [...] >>>>>> Even when taking the spinlocks, efficient iteration on active threads is >>>>>> done with for_each_cpu(cpu, mm_cpumask(current->mm)), which depends on >>>>>> the same cpumask, and thus requires the same memory barriers around the >>>>>> updates. >>>>> Ouch!!! Good point and good catch!!! >>>>> >>>>>> We could switch to an inefficient iteration on all online CPUs instead, >>>>>> and check read runqueue ->mm with the spinlock held. Is that what you >>>>>> propose ? This will cause reading of large amounts of runqueue >>>>>> information, especially on large systems running few threads. The other >>>>>> way around is to iterate on all the process threads: in this case, small >>>>>> systems running many threads will have to read information about many >>>>>> inactive threads, which is not much better. >>>>> I am not all that worried about exactly what we do as long as it is >>>>> pretty obviously correct. We can then improve performance when and as >>>>> the need arises. We might need to use any of the strategies you >>>>> propose, or perhaps even choose among them depending on the number of >>>>> threads in the process, the number of CPUs, and so forth. (I hope not, >>>>> but...) >>>>> >>>>> My guess is that an obviously correct approach would work well for a >>>>> slowpath. If someone later runs into performance problems, we can fix >>>>> them with the added knowledge of what they are trying to do. >>>>> >>>> OK, here is what I propose. Let's choose between two implementations >>>> (v3a and v3b), which implement two "obviously correct" approaches. In >>>> summary: >>>> >>>> * baseline (based on 2.6.32.2) >>>> text data bss dec hex filename >>>> 76887 8782 2044 87713 156a1 kernel/sched.o >>>> >>>> * v3a: ipi to many using mm_cpumask >>>> >>>> - adds smp_mb__before_clear_bit()/smp_mb__after_clear_bit() before and >>>> after mm_cpumask stores in context_switch(). They are only executed >>>> when oldmm and mm are different. (it's my turn to hide behind an >>>> appropriately-sized boulder for touching the scheduler). ;) Note that >>>> it's not that bad, as these barriers turn into simple compiler barrier() >>>> on: >>>> avr32, blackfin, cris, frb, h8300, m32r, m68k, mn10300, score, sh, >>>> sparc, x86 and xtensa. >>>> The less lucky architectures gaining two smp_mb() are: >>>> alpha, arm, ia64, mips, parisc, powerpc and s390. >>>> ia64 is gaining only one smp_mb() thanks to its acquire semantic. >>>> - size >>>> text data bss dec hex filename >>>> 77239 8782 2044 88065 15801 kernel/sched.o >>>> -> adds 352 bytes of text >>>> - Number of lines (system call source code, w/o comments) : 18 >>>> >>>> * v3b: iteration on min(num_online_cpus(), nr threads in the process), >>>> taking runqueue spinlocks, allocating a cpumask, ipi to many to the >>>> cpumask. Does not allocate the cpumask if only a single IPI is needed. >>>> >>>> - only adds sys_membarrier() and related functions. >>>> - size >>>> text data bss dec hex filename >>>> 78047 8782 2044 88873 15b29 kernel/sched.o >>>> -> adds 1160 bytes of text >>>> - Number of lines (system call source code, w/o comments) : 163 >>>> >>>> I'll reply to this email with the two implementations. Comments are >>>> welcome. >>> Cool!!! Just for completeness, I point out the following trivial >>> implementation: >>> >>> /* >>> * sys_membarrier - issue memory barrier on current process running threads >>> * >>> * Execute a memory barrier on all running threads of the current process. >>> * Upon completion, the caller thread is ensured that all process threads >>> * have passed through a state where memory accesses match program order. >>> * (non-running threads are de facto in such a state) >>> * >>> * Note that synchronize_sched() has the side-effect of doing a memory >>> * barrier on each CPU. >>> */ >>> SYSCALL_DEFINE0(membarrier) >>> { >>> synchronize_sched(); >>> } >>> >>> This does unnecessarily hit all CPUs in the system, but has the same >>> minimal impact that in-kernel RCU already has. It has long latency, >>> (milliseconds) which might well disqualify it from consideration for >>> some applications. On the other hand, it automatically batches multiple >>> concurrent calls to sys_membarrier(). >> Benchmarking this implementation: >> >> 1000 calls to sys_membarrier() take: >> >> T=1: 0m16.007s >> T=2: 0m16.006s >> T=3: 0m16.010s >> T=4: 0m16.008s >> T=5: 0m16.005s >> T=6: 0m16.005s >> T=7: 0m16.005s >> >> For a 16 ms per call (my HZ is 250), as you expected. So this solution >> brings a slowdown of 10,000 times compared to the IPI-based solution. >> We'd be better off using signals instead. > >>>From a latency viewpoint, yes. But synchronize_sched() consumes far > less CPU time than do signals, avoids waking up sleeping CPUs, batches > concurrent requests, and seems to be of some use in the kernel. ;-) > > But, as I said, just for completeness. > > Thanx, Paul > Actually, I like this implementation. (synchronize_sched() need be changed to synchronize_kernel_and_user_sched() or something else) IPI-implementation and signal-implementation cost too much. and this implementation just wait until things are done, very low cost. The time of kernel rcu G.P. is typically 3/HZ seconds (for all implementations except preemptable rcu). It is a large latency. but it's nothing important I think: 1) user should also call synchronize_sched() rarely. 2) If user care this latency, user can just implement a userland call_rcu userland_call_rcu() { insert rcu_head to rcu_callback_list. } rcu_callback_thread() { for (;;) { handl_list = rcu_callback_list; rcu_callback_list = NULL; userland_synchronize_sched(); handle the callback in handl_list } } 3) kernel rcu VS userland IPI-implementation RCU: userland_synchronize_sched() is less latency than kernel rcu? userland has more priority to send a lot of IPIs? It sounds crazy for me. See also this email(2010-1-11) I sent to you offlist: > /* Lai jiangshan define it for fun */ > #define synchronize_kernel_sched() synchronize_sched() > > /* We can use the current RCU code to implement one of the following */ > extern void synchronize_kernel_and_user_sched(void); > extern void synchronize_user_sched(void); > > /* > * wait until all cpu(which in userspace) enter kernel and call mb() > * (recommend) > */ > extern void synchronize_user_mb(void); > > void sys_membarrier(void) > { > /* > * 1) We add very little overhead to kernel, we just wait at kernel space. > * 2) Several processes which call sys_membarrier() wait the same *batch*. > */ > > synchronize_kernel_and_user_sched(); > /* OR synchronize_user_sched()/synchronize_user_mb() */ > } >