[RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3a)

Mathieu Desnoyers
2010-01-11T04:34:11Z

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)

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:
- simply send-to-many to the mm_cpumask. It contains the list of processors we
have to IPI to (which use the mm), and this mask is updated atomically.

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.

For my Intel Xeon E5405 (new set of results, disabled kernel debugging)

T=1: 0m18.921s
T=2: 0m19.457s
T=3: 0m21.619s
T=4: 0m21.641s
T=5: 0m23.426s
T=6: 0m26.450s
T=7: 0m27.731s

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.)

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

Results in liburcu:

Operations in 10s, 6 readers, 2 writers:

(what we previously had)
memory barriers in reader: 973494744 reads, 892368 writes
signal-based scheme: 6289946025 reads, 1251 writes

(what we have now, with dynamic sys_membarrier check)
memory barriers in reader: 907693804 reads, 817793 writes
sys_membarrier scheme: 4061976535 reads, 526807 writes

So the dynamic sys_membarrier availability check adds some overhead to the
read-side, but besides that, we can see that we are close to the read-side
performance of the signal-based scheme and also close (5/8) to the performance
of the memory-barrier write-side. We have a write-side speedup of 421:1 over the
signal-based scheme by using the sys_membarrier system call. This allows a 4.5:1
read-side speedup over the memory barrier scheme.

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 | 59 ++++++++++++++++++++++++++++++++++++++-
2 files changed, 60 insertions(+), 1 deletion(-)

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 19:21:31.000000000 -0500
+++ linux-2.6-lttng/arch/x86/include/asm/unistd_64.h 2010-01-10 19:21:37.000000000 -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 19:21:31.000000000 -0500
+++ linux-2.6-lttng/kernel/sched.c 2010-01-10 22:22:40.000000000 -0500
@@ -2861,12 +2861,26 @@ context_switch(struct rq *rq, struct tas
*/
arch_start_context_switch(prev);

+ /*
+ * sys_membarrier IPI-mb scheme requires a memory barrier between
+ * user-space thread execution and update to mm_cpumask.
+ */
+ if (likely(oldmm) && likely(oldmm != mm))
+ smp_mb__before_clear_bit();
+
if (unlikely(!mm)) {
next->active_mm = oldmm;
atomic_inc(&oldmm->mm_count);
enter_lazy_tlb(oldmm, next);
- } else
+ } else {
switch_mm(oldmm, mm, next);
+ /*
+ * sys_membarrier IPI-mb scheme requires a memory barrier
+ * between update to mm_cpumask and user-space thread execution.
+ */
+ if (likely(oldmm != mm))
+ smp_mb__after_clear_bit();
+ }

if (unlikely(!prev->mm)) {
prev->active_mm = NULL;
@@ -10822,6 +10836,49 @@ struct cgroup_subsys cpuacct_subsys = {
};
#endif /* CONFIG_CGROUP_CPUACCT */

+/*
+ * Execute a memory barrier on all active threads from the current process
+ * on SMP systems. Do not rely on implicit barriers in
+ * smp_call_function_many(), just in case they are ever relaxed in the future.
+ */
+static void membarrier_ipi(void *unused)
+{
+ smp_mb();
+}
+
+/*
+ * 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)
+ */
+SYSCALL_DEFINE0(membarrier)
+{
+#ifdef CONFIG_SMP
+ if (unlikely(thread_group_empty(current)))
+ return 0;
+ /*
+ * Memory barrier on the caller thread _before_ sending first
+ * IPI. Matches memory barriers around mm_cpumask modification in
+ * context_switch().
+ */
+ smp_mb();
+ preempt_disable();
+ smp_call_function_many(mm_cpumask(current->mm), membarrier_ipi,
+ NULL, 1);
+ preempt_enable();
+ /*
+ * Memory barrier on the caller thread _after_ we finished
+ * waiting for the last IPI. Matches memory barriers around mm_cpumask
+ * modification in context_switch().
+ */
+ smp_mb();
+#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