Messages in this thread | | | Date | Wed, 13 Jan 2010 21:13:53 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier |
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On Thu, Jan 14, 2010 at 10:56:08AM +0800, Lai Jiangshan wrote: > 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)
The global memory barriers is indeed very much a side-effect of synchronize_sched(), not its main purpose, you are right that its name is a bit strange for this purpose. ;-)
> 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
In the common case, you are correct. On the other hand, we did need to do synchronize_rcu_expedited() and friends in the kernel, so it is reasonable to expect that user-level RCU uses will also need expedited interfaces.
> 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.
You say "crazy" as if it was a bad thing. ;-)
(Sorry, couldn't resist...)
But it is important to keep in mind that sys_membarrier() is just one part of the user-level RCU implementation. When you add in the necessary waiting on per-thread counters, the user-level RCU is probably not that much cheaper than the expedited in-kernel RCU primitives.
> 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() */ > > }
If I am not getting too confused, Mathieu's latest patch does do synchronize_sched() for the non-expedited case. Mathieu pointed it out in his email of January 9th, though not as a serious suggestion, from what I can tell. Your (private) email was indeed next, so as far as I am concerned you do indeed share the credit/blame for suggesting use of synchronize_sched() as a long-latency/low-overhead implementation of sys_membarrier().
Mathieu, given that Lai has now posted publicly, could you please include at least note crediting him for the first serious suggestion of using synchronize_sched()?
Thanx, Paul
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