lkml.org 
[lkml]   [2007]   [Aug]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] SLUB: use have_arch_cmpxchg()
    On Mon, 27 Aug 2007, Mathieu Desnoyers wrote:

    > Hrm, actually, I don't think such have_arch_cmpxchg() macro will be
    > required at all because of the small performance hit disabling
    > preemption will have on the slow and fast paths. Let's compare, for each
    > of the slow path and fast path, what locking looks like on architecture
    > with and without local cmpxchg:
    >
    > What we actually do here is:
    >
    > fast path:
    > with local_cmpxchg:
    > preempt_disable()
    > preempt_enable()
    > without local_cmpxchg:
    > preempt_disable()
    > local_irq_save
    > local_irq_restore
    > preempt_enable()
    > (we therefore disable preemption _and_ disable interrupts for
    > nothing)

    Hmmm..... This is a performance issue for preempt kernels.

    > slow path:
    > both with and without local_cmpxchg():
    > preempt_disable()
    > preempt_enable()

    Here we potentially loose our per cpu structure since the process may be
    rescheduled.

    > local_irq_save()
    > local_irq_restore()
    >
    >
    > Therefore, we would add preempt disable/enable to the fast path of
    > architectures where local_cmpxchg is emulated with irqs off. But since
    > preempt disable/enable is just a check counter increment/decrement with
    > barrier() and thread flag check, I doubt it would hurt performances
    > enough to justify the added complexity of disabling interrupts for the
    > whole fast path in this case.

    One additional cacheline referenced in the hot path. Ok the counter may be
    hot as well if this is a preemptible kernel. Nevertheless a cause for
    concern.

    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-08-27 21:47    [W:4.081 / U:0.956 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site