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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jan 11, 2010 at 03:21:04PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Sun, Jan 10, 2010 at 11:25:21PM -0500, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > [...]
> > > > > Even when taking the spinlocks, efficient iteration on active threads is
> > > > > done with for_each_cpu(cpu, mm_cpumask(current->mm)), which depends on
> > > > > the same cpumask, and thus requires the same memory barriers around the
> > > > > updates.
> > > > 
> > > > Ouch!!!  Good point and good catch!!!
> > > > 
> > > > > We could switch to an inefficient iteration on all online CPUs instead,
> > > > > and check read runqueue ->mm with the spinlock held. Is that what you
> > > > > propose ? This will cause reading of large amounts of runqueue
> > > > > information, especially on large systems running few threads. The other
> > > > > way around is to iterate on all the process threads: in this case, small
> > > > > systems running many threads will have to read information about many
> > > > > inactive threads, which is not much better.
> > > > 
> > > > I am not all that worried about exactly what we do as long as it is
> > > > pretty obviously correct.  We can then improve performance when and as
> > > > the need arises.  We might need to use any of the strategies you
> > > > propose, or perhaps even choose among them depending on the number of
> > > > threads in the process, the number of CPUs, and so forth.  (I hope not,
> > > > but...)
> > > > 
> > > > My guess is that an obviously correct approach would work well for a
> > > > slowpath.  If someone later runs into performance problems, we can fix
> > > > them with the added knowledge of what they are trying to do.
> > > > 
> > > 
> > > OK, here is what I propose. Let's choose between two implementations
> > > (v3a and v3b), which implement two "obviously correct" approaches. In
> > > summary:
> > > 
> > > * baseline (based on 2.6.32.2)
> > >    text	   data	    bss	    dec	    hex	filename
> > >   76887	   8782	   2044	  87713	  156a1	kernel/sched.o
> > > 
> > > * v3a: ipi to many using mm_cpumask
> > > 
> > > - adds smp_mb__before_clear_bit()/smp_mb__after_clear_bit() before and
> > >   after mm_cpumask stores in context_switch(). They are only executed
> > >   when oldmm and mm are different. (it's my turn to hide behind an
> > >   appropriately-sized boulder for touching the scheduler). ;) Note that
> > >   it's not that bad, as these barriers turn into simple compiler barrier()
> > >   on:
> > >     avr32, blackfin, cris, frb, h8300, m32r, m68k, mn10300, score, sh,
> > >     sparc, x86 and xtensa.
> > >   The less lucky architectures gaining two smp_mb() are:
> > >     alpha, arm, ia64, mips, parisc, powerpc and s390.
> > >   ia64 is gaining only one smp_mb() thanks to its acquire semantic.
> > > - size
> > >    text	   data	    bss	    dec	    hex	filename
> > >   77239	   8782	   2044	  88065	  15801	kernel/sched.o
> > >   -> adds 352 bytes of text
> > > - Number of lines (system call source code, w/o comments) : 18
> > > 
> > > * v3b: 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.
> > > 
> > > - only adds sys_membarrier() and related functions.
> > > - size
> > >    text	   data	    bss	    dec	    hex	filename
> > >   78047	   8782	   2044	  88873	  15b29	kernel/sched.o
> > >   -> adds 1160 bytes of text
> > > - Number of lines (system call source code, w/o comments) : 163
> > > 
> > > I'll reply to this email with the two implementations. Comments are
> > > welcome.
> > 
> > Cool!!!  Just for completeness, I point out the following trivial
> > implementation:
> > 
> > /*
> >  * 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)
> >  *
> >  * Note that synchronize_sched() has the side-effect of doing a memory
> >  * barrier on each CPU.
> >  */
> > SYSCALL_DEFINE0(membarrier)
> > {
> > 	synchronize_sched();
> > }
> > 
> > This does unnecessarily hit all CPUs in the system, but has the same
> > minimal impact that in-kernel RCU already has.  It has long latency,
> > (milliseconds) which might well disqualify it from consideration for
> > some applications.  On the other hand, it automatically batches multiple
> > concurrent calls to sys_membarrier().
> 
> Benchmarking this implementation:
> 
> 1000 calls to sys_membarrier() take:
> 
> T=1: 0m16.007s
> T=2: 0m16.006s
> T=3: 0m16.010s
> T=4: 0m16.008s
> T=5: 0m16.005s
> T=6: 0m16.005s
> T=7: 0m16.005s
> 
> For a 16 ms per call (my HZ is 250), as you expected. So this solution
> brings a slowdown of 10,000 times compared to the IPI-based solution.
> We'd be better off using signals instead.

>From a latency viewpoint, yes.  But synchronize_sched() consumes far
less CPU time than do signals, avoids waking up sleeping CPUs, batches
concurrent requests, and seems to be of some use in the kernel.  ;-)

But, as I said, just for completeness.

							Thanx, Paul