lkml.org 
[lkml]   [2010]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
    * Andi Kleen (andi@firstfloor.org) wrote:
    > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> writes:
    >
    > > Both the signal-based and the sys_membarrier userspace RCU schemes
    > > permit us to remove the memory barrier from the userspace RCU
    > > rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
    > > accelerating them. These memory barriers are replaced by compiler
    > > barriers on the read-side, and all matching memory barriers on the
    > > write-side are turned into an invokation of a memory barrier on all
    > > active threads in the process. By letting the kernel perform this
    > > synchronization rather than dumbly sending a signal to every process
    > > threads (as we currently do), we diminish the number of unnecessary wake
    > > ups and only issue the memory barriers on active threads. Non-running
    > > threads do not need to execute such barrier anyway, because these are
    > > implied by the scheduler context switches.
    >
    > I'm not sure all this effort is really needed on architectures
    > with strong memory ordering.

    Do we still have many out there that support SMP ? Even newer ARM now
    need memory barriers.

    >
    > > + * The current implementation simply executes a memory barrier in an IPI handler
    > > + * on each active cpu. Going through the hassle of taking run queue locks and
    > > + * checking if the thread running on each online CPU belongs to the current
    > > + * thread seems more heavyweight than the cost of the IPI itself.
    > > + */
    > > +SYSCALL_DEFINE0(membarrier)
    > > +{
    > > + on_each_cpu(membarrier_ipi, NULL, 1);
    >
    > Can't you use mm->cpu_vm_mask?

    I'll go for PeterZ scheme, which is based on cpu_vm_mask.

    Thanks,

    Mathieu


    >
    > -Andi

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


    \
     
     \ /
      Last update: 2010-01-07 16:15    [W:0.023 / U:0.808 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site