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

Mathieu Desnoyers
2010-01-11T17:41:00Z

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Sun, Jan 10, 2010 at 11:29:03PM -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)
>
> Given that this has the memory barrier both before and after the
> assignment to ->mm, looks good to me from a memory-ordering viewpoint.
> I must defer to others on the effect on context-switch overhead.

More precisely, it's the assignment to cpu_vm_mask (clear bit/set bit)
that needs to be surrounded by memory barriers here. This is what we use
as cpu mask to which IPIs are sent. Only the current ->mm is accessed,
so ->mm ordering is not the issue here.

Thanks,

Mathieu

>
> Thanx, Paul
>
> > 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

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