Re: [PATCH v2] powerpc/numa: Fix percpu allocations to be NUMA aware

[Nicholas Piggin <npiggin@gmail.com>]

On Tue,  6 Jun 2017 20:23:57 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> In commit 8c272261194d ("powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID"), we
> switched to the generic implementation of cpu_to_node(), which uses a percpu
> variable to hold the NUMA node for each CPU.
> 
> Unfortunately we neglected to notice that we use cpu_to_node() in the allocation
> of our percpu areas, leading to a chicken and egg problem. In practice what
> happens is when we are setting up the percpu areas, cpu_to_node() reports that
> all CPUs are on node 0, so we allocate all percpu areas on node 0.
> 
> This is visible in the dmesg output, as all pcpu allocs being in group 0:
> 
>   pcpu-alloc: [0] 00 01 02 03 [0] 04 05 06 07
>   pcpu-alloc: [0] 08 09 10 11 [0] 12 13 14 15
>   pcpu-alloc: [0] 16 17 18 19 [0] 20 21 22 23
>   pcpu-alloc: [0] 24 25 26 27 [0] 28 29 30 31
>   pcpu-alloc: [0] 32 33 34 35 [0] 36 37 38 39
>   pcpu-alloc: [0] 40 41 42 43 [0] 44 45 46 47
> 
> To fix it we need an early_cpu_to_node() which can run prior to percpu being
> setup. We already have the numa_cpu_lookup_table we can use, so just plumb it
> in. With the patch dmesg output shows two groups, 0 and 1:
> 
>   pcpu-alloc: [0] 00 01 02 03 [0] 04 05 06 07
>   pcpu-alloc: [0] 08 09 10 11 [0] 12 13 14 15
>   pcpu-alloc: [0] 16 17 18 19 [0] 20 21 22 23
>   pcpu-alloc: [1] 24 25 26 27 [1] 28 29 30 31
>   pcpu-alloc: [1] 32 33 34 35 [1] 36 37 38 39
>   pcpu-alloc: [1] 40 41 42 43 [1] 44 45 46 47
> 
> We can also check the data_offset in the paca of various CPUs, with the fix we
> see:
> 
>   CPU 0:  data_offset = 0x0ffe8b0000
>   CPU 24: data_offset = 0x1ffe5b0000
> 
> And we can see from dmesg that CPU 24 has an allocation on node 1:
> 
>   node   0: [mem 0x0000000000000000-0x0000000fffffffff]
>   node   1: [mem 0x0000001000000000-0x0000001fffffffff]
> 
> Cc: stable@vger.kernel.org # v3.16+
> Fixes: 8c272261194d ("powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Looks good.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>