lkml.org 
[lkml]   [2011]   [Feb]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread
    * Steven Rostedt (rostedt@goodmis.org) wrote:
    > On Wed, 2011-02-23 at 17:16 +0100, Frederic Weisbecker wrote:
    > > On Tue, Feb 22, 2011 at 05:39:40PM -0800, Paul E. McKenney wrote:
    > > > +/*
    > > > + * Wake up the current CPU's kthread. This replaces raise_softirq()
    > > > + * in earlier versions of RCU.
    > > > + */
    > > > +static void invoke_rcu_kthread(void)
    > > > +{
    > > > + unsigned long flags;
    > > > + wait_queue_head_t *q;
    > > > + int cpu;
    > > > +
    > > > + local_irq_save(flags);
    > > > + cpu = smp_processor_id();
    > > > + if (per_cpu(rcu_cpu_kthread_task, cpu) == NULL) {
    > > > + local_irq_restore(flags);
    > > > + return;
    > > > + }
    > > > + per_cpu(rcu_cpu_has_work, cpu) = 1;
    > > > + q = &per_cpu(rcu_cpu_wq, cpu);
    > >
    > > I see you make extensive use of per_cpu() accessors even for
    > > local variables.
    > >
    > > I tend to think it's better to use __get_cpu_var() for local
    > > accesses and keep per_cpu() for remote accesses.
    > >
    > > There are several reasons for that:
    > >
    > > * __get_cpu_var() checks we are in a non-preemptible section,
    > > per_cpu() doesn't. That may sound of a limited interest for code like the
    > > above, but by the time code can move, and then we might lose track of some
    > > things, etc...
    >
    > Ah, but so does smp_processor_id() ;-)
    >
    > >
    > > * local accesses can be optimized by architectures. per_cpu() implies
    > > finding the local cpu number, and dereference an array of cpu offsets with
    > > that number to find the local cpu offset.
    > > __get_cpu_var() does a direct access to __my_cpu_offset which is a nice
    > > shortcut if the arch implements it.

    [Adding Christoph Lameter to CC list]

    This is not quite true on x86_64 and s390 anymore. __get_cpu_var() now
    uses a segment selector override to get the local CPU variable on x86.
    See x86's percpu.h for details.

    So even performance-wise, using __get_cpu_var() over per_cpu() should be
    a win on widely used architectures nowadays, thanks to Christoph's work
    on this_cpu accessors.

    >
    > True, but we could also argue that the multiple checks for being preempt
    > can also be an issue.

    At least on x86 preemption don't actually need to be disabled: selection
    of the right per-cpu memory location is done atomically with the rest of
    the instruction by the segment selector.

    >
    > >
    > > * It makes code easier to review: we know that __get_cpu_var() is
    > > for local accesses and per_cpu() for remote.
    >
    > This I'll agree with you.
    >
    > In the past, I've thought about which one is better (per_cpu() vs
    > __get_cpu_var()).
    >
    > But, that last point is a good one. Just knowing that this is for the
    > local CPU helps with the amount of info your mind needs to process when
    > looking at this code. And the mind needs all the help it can get when
    > reviewing this code ;-)
    >

    Agreed, better documentation of the code is also a win.

    Thanks,

    Mathieu

    > -- Steve
    >
    > >
    > > > + wake_up(q);
    > > > + local_irq_restore(flags);
    > > > +}
    >
    >

    --
    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
    http://www.efficios.com


    \
     
     \ /
      Last update: 2011-02-23 18:07    [W:0.062 / U:0.744 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site