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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b)
    * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
    > On Sun, Jan 10, 2010 at 11:30:16PM -0500, Mathieu Desnoyers wrote:
    > > Here is an implementation of a new system call, sys_membarrier(), which
    > > executes a memory barrier on all threads of the current process.
    > >
    > > It aims at greatly simplifying and enhancing the current signal-based
    > > liburcu userspace RCU synchronize_rcu() implementation.
    > > (found at http://lttng.org/urcu)
    >
    > I didn't expect quite this comprehensive of an implementation from the
    > outset, but I guess I cannot complain. ;-)
    >
    > Overall, good stuff.
    >
    > Interestingly enough, what you have implemented is analogous to
    > synchronize_rcu_expedited() and friends that have recently been added
    > to the in-kernel RCU API. By this analogy, my earlier semi-suggestion
    > of synchronize_rcu(0 would be a candidate non-expedited implementation.
    > Long latency, but extremely low CPU consumption, full batching of
    > concurrent requests (even unrelated ones), and so on.

    Yes, the main different I think is that the sys_membarrier
    infrastructure focuses on IPI-ing only the current process running
    threads.

    >
    > A few questions interspersed below.
    >
    > > Changelog since v1:
    > >
    > > - Only perform the IPI in CONFIG_SMP.
    > > - Only perform the IPI if the process has more than one thread.
    > > - Only send IPIs to CPUs involved with threads belonging to our process.
    > > - Adaptative IPI scheme (single vs many IPI with threshold).
    > > - Issue smp_mb() at the beginning and end of the system call.
    > >
    > > Changelog since v2:
    > >
    > > - 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.
    > >
    > >
    > > Both the signal-based and the sys_membarrier userspace RCU schemes
    > > permit us to remove the memory barrier from the userspace RCU
    > > rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
    > > accelerating them. These memory barriers are replaced by compiler
    > > barriers on the read-side, and all matching memory barriers on the
    > > write-side are turned into an invokation of a memory barrier on all
    > > active threads in the process. By letting the kernel perform this
    > > synchronization rather than dumbly sending a signal to every process
    > > threads (as we currently do), we diminish the number of unnecessary wake
    > > ups and only issue the memory barriers on active threads. Non-running
    > > threads do not need to execute such barrier anyway, because these are
    > > implied by the scheduler context switches.
    > >
    > > To explain the benefit of this scheme, let's introduce two example threads:
    > >
    > > Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
    > > Thread B (frequent, e.g. executing liburcu rcu_read_lock()/rcu_read_unlock())
    > >
    > > In a scheme where all smp_mb() in thread A synchronize_rcu() are
    > > ordering memory accesses with respect to smp_mb() present in
    > > rcu_read_lock/unlock(), we can change all smp_mb() from
    > > synchronize_rcu() into calls to sys_membarrier() and all smp_mb() from
    > > rcu_read_lock/unlock() into compiler barriers "barrier()".
    > >
    > > Before the change, we had, for each smp_mb() pairs:
    > >
    > > Thread A Thread B
    > > prev mem accesses prev mem accesses
    > > smp_mb() smp_mb()
    > > follow mem accesses follow mem accesses
    > >
    > > After the change, these pairs become:
    > >
    > > Thread A Thread B
    > > prev mem accesses prev mem accesses
    > > sys_membarrier() barrier()
    > > follow mem accesses follow mem accesses
    > >
    > > As we can see, there are two possible scenarios: either Thread B memory
    > > accesses do not happen concurrently with Thread A accesses (1), or they
    > > do (2).
    > >
    > > 1) Non-concurrent Thread A vs Thread B accesses:
    > >
    > > Thread A Thread B
    > > prev mem accesses
    > > sys_membarrier()
    > > follow mem accesses
    > > prev mem accesses
    > > barrier()
    > > follow mem accesses
    > >
    > > In this case, thread B accesses will be weakly ordered. This is OK,
    > > because at that point, thread A is not particularly interested in
    > > ordering them with respect to its own accesses.
    > >
    > > 2) Concurrent Thread A vs Thread B accesses
    > >
    > > Thread A Thread B
    > > prev mem accesses prev mem accesses
    > > sys_membarrier() barrier()
    > > follow mem accesses follow mem accesses
    > >
    > > In this case, thread B accesses, which are ensured to be in program
    > > order thanks to the compiler barrier, will be "upgraded" to full
    > > smp_mb() thanks to the IPIs executing memory barriers on each active
    > > system threads. Each non-running process threads are intrinsically
    > > serialized by the scheduler.
    > >
    > > Just tried with a cache-hot kernel compilation using 6/8 CPUs.
    > >
    > > Normally: real 2m41.852s
    > > With the sys_membarrier+1 busy-looping thread running: real 5m41.830s
    > >
    > > So... 2x slower. That hurts.
    > >
    > > So let's try allocating a cpu mask for PeterZ scheme. I prefer to have a
    > > small allocation overhead and benefit from cpumask broadcast if
    > > possible so we scale better. But that all depends on how big the
    > > allocation overhead is.
    > >
    > > Impact of allocating a cpumask (time for 10,000,000 sys_membarrier
    > > calls, one thread is doing the sys_membarrier, the others are busy
    > > looping)). Given that it costs almost half as much to perform the
    > > cpumask allocation than to send a single IPI, as we iterate on the CPUs
    > > until we find more than N match or iterated on all cpus. If we only have
    > > N match or less, we send single IPIs. If we need more than that, then we
    > > switch to the cpumask allocation and send a broadcast IPI to the cpumask
    > > we construct for the matching CPUs. Let's call it the "adaptative IPI
    > > scheme".
    > >
    > > For my Intel Xeon E5405
    > >
    > > *This is calibration only, not taking the runqueue locks*
    > >
    > > Just doing local mb()+single IPI to T other threads:
    > >
    > > T=1: 0m18.801s
    > > T=2: 0m29.086s
    > > T=3: 0m46.841s
    > > T=4: 0m53.758s
    > > T=5: 1m10.856s
    > > T=6: 1m21.142s
    > > T=7: 1m38.362s
    > >
    > > Just doing cpumask alloc+IPI-many to T other threads:
    > >
    > > T=1: 0m21.778s
    > > T=2: 0m22.741s
    > > T=3: 0m22.185s
    > > T=4: 0m24.660s
    > > T=5: 0m26.855s
    > > T=6: 0m30.841s
    > > T=7: 0m29.551s
    > >
    > > So I think the right threshold should be 1 thread (assuming other
    > > architecture will behave like mine). So starting with 2 threads, we
    > > allocate the cpumask before sending IPIs.
    > >
    > > *end of calibration*
    > >
    > > Resulting adaptative scheme, with runqueue locks:
    > >
    > > T=1: 0m20.990s
    > > T=2: 0m22.588s
    > > T=3: 0m27.028s
    > > T=4: 0m29.027s
    > > T=5: 0m32.592s
    > > T=6: 0m36.556s
    > > T=7: 0m33.093s
    > >
    > > The expected top pattern, when using 1 CPU for a thread doing sys_membarrier()
    > > in a loop and other threads busy-waiting in user-space on a variable shows that
    > > the thread doing sys_membarrier is doing mostly system calls, and other threads
    > > are mostly running in user-space. Side-note, in this test, it's important to
    > > check that individual threads are not always fully at 100% user-space time (they
    > > range between ~95% and 100%), because when some thread in the test is always at
    > > 100% on the same CPU, this means it does not get the IPI at all. (I actually
    > > found out about a bug in my own code while developing it with this test.)
    >
    > The below data is for how many threads in the process?

    8 threads: one doing sys_membarrier() in a loop, 7 others waiting on a
    variable.

    > Also, is "top"
    > accurate given that the IPI handler will have interrupts disabled?

    Probably not. AFAIK. "top" does not really consider interrupts into its
    accounting. So, better take this top output with a grain of salt or two.

    >
    > > Cpu0 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > Cpu1 : 99.7%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.3%hi, 0.0%si, 0.0%st
    > > Cpu2 : 99.3%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.7%hi, 0.0%si, 0.0%st
    > > Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > Cpu4 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > Cpu5 : 96.0%us, 1.3%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 2.6%si, 0.0%st
    > > Cpu6 : 1.3%us, 98.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > Cpu7 : 96.1%us, 3.3%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.3%hi, 0.3%si, 0.0%st
    > >
    > > The system call number is only assigned for x86_64 in this RFC patch.
    > >
    > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    > > CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    > > CC: mingo@elte.hu
    > > CC: laijs@cn.fujitsu.com
    > > CC: dipankar@in.ibm.com
    > > CC: akpm@linux-foundation.org
    > > CC: josh@joshtriplett.org
    > > CC: dvhltc@us.ibm.com
    > > CC: niv@us.ibm.com
    > > CC: tglx@linutronix.de
    > > CC: peterz@infradead.org
    > > CC: rostedt@goodmis.org
    > > CC: Valdis.Kletnieks@vt.edu
    > > CC: dhowells@redhat.com
    > > ---
    > > arch/x86/include/asm/unistd_64.h | 2
    > > kernel/sched.c | 219 +++++++++++++++++++++++++++++++++++++++
    > > 2 files changed, 221 insertions(+)
    > >
    > > Index: linux-2.6-lttng/arch/x86/include/asm/unistd_64.h
    > > ===================================================================
    > > --- linux-2.6-lttng.orig/arch/x86/include/asm/unistd_64.h 2010-01-10 22:23:59.000000000 -0500
    > > +++ linux-2.6-lttng/arch/x86/include/asm/unistd_64.h 2010-01-10 22:29:30.000000000 -0500
    > > @@ -661,6 +661,8 @@ __SYSCALL(__NR_pwritev, sys_pwritev)
    > > __SYSCALL(__NR_rt_tgsigqueueinfo, sys_rt_tgsigqueueinfo)
    > > #define __NR_perf_event_open 298
    > > __SYSCALL(__NR_perf_event_open, sys_perf_event_open)
    > > +#define __NR_membarrier 299
    > > +__SYSCALL(__NR_membarrier, sys_membarrier)
    > >
    > > #ifndef __NO_STUBS
    > > #define __ARCH_WANT_OLD_READDIR
    > > Index: linux-2.6-lttng/kernel/sched.c
    > > ===================================================================
    > > --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-10 22:23:59.000000000 -0500
    > > +++ linux-2.6-lttng/kernel/sched.c 2010-01-10 23:12:35.000000000 -0500
    > > @@ -119,6 +119,11 @@
    > > */
    > > #define RUNTIME_INF ((u64)~0ULL)
    > >
    > > +/*
    > > + * IPI vs cpumask broadcast threshold. Threshold of 1 IPI.
    > > + */
    > > +#define ADAPT_IPI_THRESHOLD 1
    > > +
    > > static inline int rt_policy(int policy)
    > > {
    > > if (unlikely(policy == SCHED_FIFO || policy == SCHED_RR))
    > > @@ -10822,6 +10827,220 @@ struct cgroup_subsys cpuacct_subsys = {
    > > };
    > > #endif /* CONFIG_CGROUP_CPUACCT */
    > >
    > > +/*
    > > + * Execute a memory barrier on all CPUs on SMP systems.
    > > + * Do not rely on implicit barriers in smp_call_function(), just in case they
    > > + * are ever relaxed in the future.
    > > + */
    > > +static void membarrier_ipi(void *unused)
    > > +{
    > > + smp_mb();
    > > +}
    > > +
    > > +/*
    > > + * Handle out-of-mem by sending per-cpu IPIs instead.
    > > + */
    >
    > Good handling for out-of-memory errors!
    >
    > > +static void membarrier_cpus_retry(int this_cpu)
    > > +{
    > > + struct mm_struct *mm;
    > > + int cpu;
    > > +
    > > + for_each_online_cpu(cpu) {
    > > + if (unlikely(cpu == this_cpu))
    > > + continue;
    > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > + mm = cpu_curr(cpu)->mm;
    > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > + if (current->mm == mm)
    > > + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
    >
    > There is of course some possibility of interrupting a real-time task,
    > as the destination CPU could context-switch once we drop the ->lock.
    > Not a criticism, just something to keep in mind. After all, the only ways
    > I can think of to avoid this possibility do so by keeping the CPU from
    > switching to the real-time task, which sort of defeats the purpose. ;-)

    Absolutely. And it's of no use to add a check within the IPI handler to
    verify if it was indeed needed, because all we would skip is a simple
    smp_mb(), which is relatively minor in terms of overhead compared to the
    IPI itself.

    >
    > > + }
    > > +}
    > > +
    > > +static void membarrier_threads_retry(int this_cpu)
    > > +{
    > > + struct mm_struct *mm;
    > > + struct task_struct *t;
    > > + struct rq *rq;
    > > + int cpu;
    > > +
    > > + list_for_each_entry_rcu(t, &current->thread_group, thread_group) {
    > > + local_irq_disable();
    > > + rq = __task_rq_lock(t);
    > > + mm = rq->curr->mm;
    > > + cpu = rq->cpu;
    > > + __task_rq_unlock(rq);
    > > + local_irq_enable();
    > > + if (cpu == this_cpu)
    > > + continue;
    > > + if (current->mm == mm)
    > > + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
    >
    > Ditto.
    >
    > > + }
    > > +}
    > > +
    > > +static void membarrier_cpus(int this_cpu)
    > > +{
    > > + int cpu, i, cpu_ipi[ADAPT_IPI_THRESHOLD], nr_cpus = 0;
    > > + cpumask_var_t tmpmask;
    > > + struct mm_struct *mm;
    > > +
    > > + /* Get CPU IDs up to threshold */
    > > + for_each_online_cpu(cpu) {
    > > + if (unlikely(cpu == this_cpu))
    > > + continue;
    >
    > OK, the above "if" handles the single-threaded-process case.
    >

    No. See

    + if (unlikely(thread_group_empty(current)))
    + return 0;

    in the caller below. The if you present here simply ensures that we
    don't do a superfluous function call on the current thread. It's
    probably not really worth it for a slow path though.

    > The UP-kernel case is handled by the #ifdef in sys_membarrier(), though
    > with a bit larger code footprint than the embedded guys would probably
    > prefer. (Or is the compiler smart enough to omit these function given no
    > calls to them? If not, recommend putting them under CONFIG_SMP #ifdef.)

    Hrm, that's a bit odd. I agree that UP systems could simply return
    -ENOSYS for sys_membarrier, but then I wonder how userland could
    distinguish between:

    - an old kernel not supporting sys_membarrier()
    -> in this case we need to use the smp_mb() fallback on the read-side
    and in synchronize_rcu().
    - a recent kernel supporting sys_membarrier(), CONFIG_SMP
    -> can use the barrier() on read-side, call sys_membarrier upon
    update.
    - a recent kernel supporting sys_membarrier, !CONFIG_SMP
    -> calls to sys_membarrier() are not required, nor is barrier().

    Or maybe we just postpone the userland smp_mb() question to another
    thread. This will eventually need to be addressed anyway. Maybe with a
    vgetmaxcpu() vsyscall.

    >
    > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > + mm = cpu_curr(cpu)->mm;
    > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > + if (current->mm == mm) {
    > > + if (nr_cpus == ADAPT_IPI_THRESHOLD) {
    > > + nr_cpus++;
    > > + break;
    > > + }
    > > + cpu_ipi[nr_cpus++] = cpu;
    > > + }
    > > + }
    > > + if (likely(nr_cpus <= ADAPT_IPI_THRESHOLD)) {
    > > + for (i = 0; i < nr_cpus; i++) {
    > > + smp_call_function_single(cpu_ipi[i],
    > > + membarrier_ipi,
    > > + NULL, 1);
    > > + }
    > > + } else {
    > > + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) {
    > > + membarrier_cpus_retry(this_cpu);
    > > + return;
    > > + }
    > > + for (i = 0; i < ADAPT_IPI_THRESHOLD; i++)
    > > + cpumask_set_cpu(cpu_ipi[i], tmpmask);
    > > + /* Continue previous online cpu iteration */
    > > + cpumask_set_cpu(cpu, tmpmask);
    > > + for (;;) {
    > > + cpu = cpumask_next(cpu, cpu_online_mask);
    > > + if (unlikely(cpu == this_cpu))
    > > + continue;
    > > + if (unlikely(cpu >= nr_cpu_ids))
    > > + break;
    > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > + mm = cpu_curr(cpu)->mm;
    > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > + if (current->mm == mm)
    > > + cpumask_set_cpu(cpu, tmpmask);
    > > + }
    > > + smp_call_function_many(tmpmask, membarrier_ipi, NULL, 1);
    > > + free_cpumask_var(tmpmask);
    > > + }
    > > +}
    > > +
    > > +static void membarrier_threads(int this_cpu)
    > > +{
    > > + int cpu, i, cpu_ipi[ADAPT_IPI_THRESHOLD], nr_cpus = 0;
    > > + cpumask_var_t tmpmask;
    > > + struct mm_struct *mm;
    > > + struct task_struct *t;
    > > + struct rq *rq;
    > > +
    > > + /* Get CPU IDs up to threshold */
    > > + list_for_each_entry_rcu(t, &current->thread_group,
    > > + thread_group) {
    > > + local_irq_disable();
    > > + rq = __task_rq_lock(t);
    > > + mm = rq->curr->mm;
    > > + cpu = rq->cpu;
    > > + __task_rq_unlock(rq);
    > > + local_irq_enable();
    > > + if (cpu == this_cpu)
    > > + continue;
    > > + if (current->mm == mm) {
    >
    > I do not believe that the above test is gaining you anything. It would
    > fail only if the task switched since the __task_rq_unlock(), but then
    > again, it could switch immediately after the above test just as well.

    OK. Anyway I think I'll go the the shorter implementation using the
    mm_cpumask, and add an additionnal ->mm check with spinlocks.

    >
    > > + if (nr_cpus == ADAPT_IPI_THRESHOLD) {
    > > + nr_cpus++;
    > > + break;
    > > + }
    > > + cpu_ipi[nr_cpus++] = cpu;
    > > + }
    > > + }
    > > + if (likely(nr_cpus <= ADAPT_IPI_THRESHOLD)) {
    > > + for (i = 0; i < nr_cpus; i++) {
    > > + smp_call_function_single(cpu_ipi[i],
    > > + membarrier_ipi,
    > > + NULL, 1);
    > > + }
    > > + } else {
    > > + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) {
    > > + membarrier_threads_retry(this_cpu);
    > > + return;
    > > + }
    > > + for (i = 0; i < ADAPT_IPI_THRESHOLD; i++)
    > > + cpumask_set_cpu(cpu_ipi[i], tmpmask);
    > > + /* Continue previous thread iteration */
    > > + cpumask_set_cpu(cpu, tmpmask);
    > > + list_for_each_entry_continue_rcu(t,
    > > + &current->thread_group,
    > > + thread_group) {
    > > + local_irq_disable();
    > > + rq = __task_rq_lock(t);
    > > + mm = rq->curr->mm;
    > > + cpu = rq->cpu;
    > > + __task_rq_unlock(rq);
    > > + local_irq_enable();
    > > + if (cpu == this_cpu)
    > > + continue;
    > > + if (current->mm == mm)
    >
    > Ditto.
    >
    > > + cpumask_set_cpu(cpu, tmpmask);
    > A> + }
    > > + smp_call_function_many(tmpmask, membarrier_ipi, NULL, 1);
    > > + free_cpumask_var(tmpmask);
    > > + }
    > > +}
    > > +
    > > +/*
    > > + * 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)
    > > + *
    > > + * We do not use mm_cpumask because there is no guarantee that each architecture
    > > + * switch_mm issues a smp_mb() before and after mm_cpumask modification upon
    > > + * scheduling change. Furthermore, leave_mm is also modifying the mm_cpumask (at
    > > + * least on x86) from the TLB flush IPI handler. So rather than playing tricky
    > > + * games with lazy TLB flush, let's simply iterate on online cpus/thread group,
    > > + * whichever is the smallest.
    > > + */
    > > +SYSCALL_DEFINE0(membarrier)
    > > +{
    > > +#ifdef CONFIG_SMP
    > > + int this_cpu;
    > > +
    > > + if (unlikely(thread_group_empty(current)))
    > > + return 0;
    > > +
    > > + rcu_read_lock(); /* protect cpu_curr(cpu)-> and rcu list */
    > > + preempt_disable();
    >
    > Hmmm... You are going to hate me for pointing this out, Mathieu, but
    > holding preempt_disable() across the whole sys_membarrier() processing
    > might be hurting real-time latency more than would unconditionally
    > IPIing all the CPUs. :-/

    Hehe, I pointed this out myself a few emails ago :) This is why I
    started by using raw_smp_processor_id(). Well, let's make it simple
    first, and then we can improve if needed.

    >
    > That said, we have no shortage of situations where we scan the CPUs with
    > preemption disabled, and with interrupts disabled, for that matter.

    Yep.

    Thanks,

    Mathieu

    >
    > > + /*
    > > + * Memory barrier on the caller thread _before_ sending first IPI.
    > > + */
    > > + smp_mb();
    > > + /*
    > > + * We don't need to include ourself in IPI, as we already
    > > + * surround our execution with memory barriers.
    > > + */
    > > + this_cpu = smp_processor_id();
    > > + /* Approximate which is fastest: CPU or thread group iteration ? */
    > > + if (num_online_cpus() <= atomic_read(&current->mm->mm_users))
    > > + membarrier_cpus(this_cpu);
    > > + else
    > > + membarrier_threads(this_cpu);
    > > + /*
    > > + * Memory barrier on the caller thread _after_ we finished
    > > + * waiting for the last IPI.
    > > + */
    > > + smp_mb();
    > > + preempt_enable();
    > > + rcu_read_unlock();
    > > +#endif /* #ifdef CONFIG_SMP */
    > > + return 0;
    > > +}
    > > +
    > > #ifndef CONFIG_SMP
    > >
    > > int rcu_expedited_torture_stats(char *page)
    > > --
    > > Mathieu Desnoyers
    > > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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