lkml.org 
[lkml]   [2010]   [Aug]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 03/10] Use percpu stats
    From
    Date
    Le mardi 31 août 2010 à 16:31 -0400, Nitin Gupta a écrit :
    > On Mon, Aug 30, 2010 at 12:20 PM, Christoph Lameter <cl@linux.com> wrote:
    > > On Mon, 9 Aug 2010, Nitin Gupta wrote:
    > >
    > >> -static void zram_stat_inc(u32 *v)
    > >> +static void zram_add_stat(struct zram *zram,
    > >> + enum zram_stats_index idx, s64 val)
    > >> {
    > >> - *v = *v + 1;
    > >> + struct zram_stats_cpu *stats;
    > >> +
    > >> + preempt_disable();
    > >> + stats = __this_cpu_ptr(zram->stats);
    > >> + u64_stats_update_begin(&stats->syncp);
    > >> + stats->count[idx] += val;
    > >> + u64_stats_update_end(&stats->syncp);
    > >> + preempt_enable();
    > >
    > > Maybe do
    > >
    > > #define zram_add_stat(zram, index, val)
    > > this_cpu_add(zram->stats->count[index], val)
    > >
    > > instead? It creates an add in a single "atomic" per cpu instruction and
    > > deals with the fallback scenarios for processors that cannot handle 64
    > > bit adds.
    > >
    > >
    >
    > Yes, this_cpu_add() seems sufficient. I can't recall why I used u64_stats_*
    > but if it's not required for atomic access to 64-bit then why was it added to
    > the mainline in the first place?

    Because we wanted to have fast 64bit counters, even on 32bit arches, and
    this has litle to do with 'atomic' on one entity, but a group of
    counters. (check drivers/net/loopback.c, lines 91-94). No lock prefix
    used in fast path.

    We also wanted readers to read correct values, not a value being changed
    by a writer, with inconsistent 32bit halves. SNMP applications want
    monotonically increasing counters.

    this_cpu_add()/this_cpu_read() doesnt fit.

    Even for single counter, this_cpu_read(64bit) is not using an RMW
    (cmpxchg8) instruction, so you can get very strange results when low
    order 32bit wraps.



    --
    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: 2010-08-31 23:31    [W:0.025 / U:4.540 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site