Commit be852795 authored Mar 04, 2008 by Eric Dumazet Committed by Linus Torvalds Mar 04, 2008

alloc_percpu() fails to allocate percpu data



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>

parent e3892296

mm/allocpercpu.c

+14 −1

Original line number	Diff line number	Diff line
		@@ -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))

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