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authorEric Dumazet <dada1@cosmosbay.com>2008-03-28 14:42:43 -0400
committerChris Wright <chrisw@sous-sol.org>2008-04-18 18:53:21 -0700
commit2f95fda842dd607e2d02f973b22a5aacf78bbd1b (patch)
tree81d6890bd4b08c082b61db48ed8d93ae5466e3e9
parent28680bfb8269703def997e2269caf9bfe2de489c (diff)
alloc_percpu() fails to allocate percpu data
upstream commit: be852795e1c8d3829ddf3cb1ce806113611fa555 Some oprofile results obtained while using tbench on a 2x2 cpu machine were very surprising. For example, loopback_xmit() function was using high number of cpu cycles to perform the statistic updates, supposed to be real cheap since they use percpu data pcpu_lstats = netdev_priv(dev); lb_stats = per_cpu_ptr(pcpu_lstats, smp_processor_id()); lb_stats->packets++; /* HERE : serious contention */ lb_stats->bytes += skb->len; struct pcpu_lstats is a small structure containing two longs. It appears that on my 32bits platform, alloc_percpu(8) allocates a single cache line, instead of giving to each cpu a separate cache line. Using the following patch gave me impressive boost in various benchmarks ( 6 % in tbench) (all percpu_counters hit this bug too) Long term fix (ie >= 2.6.26) would be to let each CPU allocate their own block of memory, so that we dont need to roudup sizes to L1_CACHE_BYTES, or merging the SGI stuff of course... Note : SLUB vs SLAB is important here to *show* the improvement, since they dont have the same minimum allocation sizes (8 bytes vs 32 bytes). This could very well explain regressions some guys reported when they switched to SLUB. Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Chris Wright <chrisw@sous-sol.org>
-rw-r--r--mm/allocpercpu.c15
1 files changed, 14 insertions, 1 deletions
diff --git a/mm/allocpercpu.c b/mm/allocpercpu.c
index 7e58322b7134..b0012e27fea8 100644
--- a/mm/allocpercpu.c
+++ b/mm/allocpercpu.c
@@ -6,6 +6,10 @@
#include <linux/mm.h>
#include <linux/module.h>
+#ifndef cache_line_size
+#define cache_line_size() L1_CACHE_BYTES
+#endif
+
/**
* percpu_depopulate - depopulate per-cpu data for given cpu
* @__pdata: per-cpu data to depopulate
@@ -52,6 +56,11 @@ void *percpu_populate(void *__pdata, size_t size, gfp_t gfp, int cpu)
struct percpu_data *pdata = __percpu_disguise(__pdata);
int node = cpu_to_node(cpu);
+ /*
+ * We should make sure each CPU gets private memory.
+ */
+ size = roundup(size, cache_line_size());
+
BUG_ON(pdata->ptrs[cpu]);
if (node_online(node))
pdata->ptrs[cpu] = kmalloc_node(size, gfp|__GFP_ZERO, node);
@@ -98,7 +107,11 @@ EXPORT_SYMBOL_GPL(__percpu_populate_mask);
*/
void *__percpu_alloc_mask(size_t size, gfp_t gfp, cpumask_t *mask)
{
- void *pdata = kzalloc(nr_cpu_ids * sizeof(void *), gfp);
+ /*
+ * We allocate whole cache lines to avoid false sharing
+ */
+ size_t sz = roundup(nr_cpu_ids * sizeof(void *), cache_line_size());
+ void *pdata = kzalloc(sz, gfp);
void *__pdata = __percpu_disguise(pdata);
if (unlikely(!pdata))