Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b)

Paul E. McKenney
2010-01-11T22:44:00Z

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.

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

> 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. ;-)

> + }
> +}
> +
> +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.

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.)

> + 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.

> + 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. :-/

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

> + /*
> + * 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