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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
    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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