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

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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@xxxxxxxxxx>
> > CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > CC: mingo@xxxxxxx
> > CC: laijs@xxxxxxxxxxxxxx
> > CC: dipankar@xxxxxxxxxx
> > CC: akpm@xxxxxxxxxxxxxxxxxxxx
> > CC: josh@xxxxxxxxxxxxxxxx
> > CC: dvhltc@xxxxxxxxxx
> > CC: niv@xxxxxxxxxx
> > CC: tglx@xxxxxxxxxxxxx
> > CC: peterz@xxxxxxxxxxxxx
> > CC: rostedt@xxxxxxxxxxx
> > CC: Valdis.Kletnieks@xxxxxx
> > CC: dhowells@xxxxxxxxxx
> > ---
> >  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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