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


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