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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Wed, Jan 06, 2010 at 11:40:07PM -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)
> > 
> > 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.
> > 
> > The current implementation simply executes a memory barrier in an IPI
> > handler on each active cpu. Going through the hassle of taking run queue
> > locks and checking if the thread running on each online CPU belongs to
> > the current thread seems more heavyweight than the cost of the IPI
> > itself (not measured though).
> > 
> > The system call number is only assigned for x86_64 in this RFC patch.
> 
> Beats the heck out of user-mode signal handlers!!!  And it is hard
> to imagine groveling through runqueues ever being a win, even on very
> large systems.  The only reasonable optimization I can imagine is to
> turn this into a no-op for a single-threaded process, but there are
> other ways to do that optimization.
> 

I'll cook something using thread_group_empty(current) for the next
version.

Thanks !

Mathieu

> Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> > 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                   |   30 ++++++++++++++++++++++++++++++
> >  2 files changed, 32 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	2010-01-06 22:11:32.000000000 -0500
> > +++ linux-2.6-lttng/arch/x86/include/asm/unistd_64.h	2010-01-06 22:11:50.000000000 -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	2010-01-06 22:11:32.000000000 -0500
> > +++ linux-2.6-lttng/kernel/sched.c	2010-01-06 23:20:42.000000000 -0500
> > @@ -10822,6 +10822,36 @@ 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();
> > +}
> > +
> > +/*
> > + * 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)
> > + *
> > + * The current implementation simply executes a memory barrier in an IPI handler
> > + * on each active cpu. Going through the hassle of taking run queue locks and
> > + * checking if the thread running on each online CPU belongs to the current
> > + * thread seems more heavyweight than the cost of the IPI itself.
> > + */
> > +SYSCALL_DEFINE0(membarrier)
> > +{
> > +	on_each_cpu(membarrier_ipi, NULL, 1);
> > +
> > +	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