Messages in this thread | | | Subject | Re: [PATCH 03/10] Use percpu stats | From | Eric Dumazet <> | Date | Tue, 31 Aug 2010 23:28:26 +0200 |
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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.
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