Re: [PATCH] alloc_percpu() fails to allocate percpu data

[Mike Snitzer]

On 3/11/08, Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
> Mike Snitzer a écrit :
>
> > On 2/21/08, Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
>  >
>  >> 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 stuffof
>  >>  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.
>  >>
>  >
>  >
>  > I see that this fix was committed to mainline as commit
>  > be852795e1c8d3829ddf3cb1ce806113611fa555
>  >
>  > The commit didn't "Cc: <stable@xxxxxxxxxx>", and it doesn't appear to
>  > be queued for 2.6.24.x.  Should it be?
>  >
>  >
>
> Yes, it should be queued fo 2.6.24.x

That means both of the following commits need to be cherry-picked into 2.6.24.x:
b3242151906372f30f57feaa43b4cac96a23edb1
be852795e1c8d3829ddf3cb1ce806113611fa555

>  > If I understand you correctly, SLAB doesn't create this particular
>  > cache thrashing on 32bit systems?  Is SLAB ok on other architectures
>  > too?  Can you (or others) comment on the importance of this fix
>  > relative to x86_64 (64byte cacheline) and SLAB?
>  >
>  >
>
>
> Fix is important both for 32 and 64 bits kernels, SLAB or SLUB.
>
>  SLAB does have this problem, but less prevalent than SLUB, because these
>  allocators dont have the same minimal size allocation (32 vs 8)
>
>  So with SLUB, it is possible that 8 CPUS share the same 64 bytes
>  cacheline to store their percpu counters, while only 2 cpus can share
>  this same cache line with SLAB allocator.

Thanks for the clarification.
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