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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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