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

Mathieu Desnoyers
2010-01-11T20:21:10Z

* 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.

Thanks,

Mathieu

>
> Thanx, Paul

--
Mathieu Desnoyers
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