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

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> On Sun, Jan 10, 2010 at 11:25:21PM -0500, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) 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.

Thanks,

Mathieu


> 
> 							Thanx, Paul

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