lkml.org 
[lkml]   [2008]   [Aug]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic on 32 bit architecture
    On Wed, Aug 27, 2008 at 02:22:50PM -0700, Andrew Morton wrote:
    > On Wed, 27 Aug 2008 23:01:52 +0200
    > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
    >
    > > >
    > > > > +static inline s64 percpu_counter_read(struct percpu_counter *fbc)
    > > > > +{
    > > > > + return fbc_count(fbc);
    > > > > +}
    > > >
    > > > This change means that a percpu_counter_read() from interrupt context
    > > > on a 32-bit machine is now deadlockable, whereas it previously was not
    > > > deadlockable on either 32-bit or 64-bit.
    > > >
    > > > This flows on to the lib/proportions.c, which uses
    > > > percpu_counter_read() and also does spin_lock_irqsave() internally,
    > > > indicating that it is (or was) designed to be used in IRQ contexts.
    > >
    > > percpu_counter() never was irq safe, which is why the proportion stuff
    > > does all the irq disabling bits by hand.
    >
    > percpu_counter_read() was irq-safe. That changes here. Needs careful
    > review, changelogging and, preferably, runtime checks. But perhaps
    > they should be inside some CONFIG_thing which won't normally be done in
    > production.
    >
    > otoh, percpu_counter_read() is in fact a rare operation, so a bit of
    > overhead probably won't matter.
    >
    > (write-often, read-rarely is the whole point. This patch's changelog's
    > assertion that "Since fbc->count is read more frequently and updated
    > rarely" is probably wrong. Most percpu_counters will have their
    > fbc->count modified far more frequently than having it read from).

    we may actually be doing percpu_counter_add. But that doesn't update
    fbc->count. Only if the local percpu values cross FBC_BATCH we update
    fbc->count. If we are modifying fbc->count more frequently than
    reading fbc->count then i guess we would be contenting of fbc->lock more.


    -aneesh


    \
     
     \ /
      Last update: 2008-08-28 05:55    [W:0.057 / U:30.576 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site