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

[Nicholas Piggin <npiggin@gmail.com>]

On Fri,  2 Jun 2017 15:14:47 +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]

Nice bug :)

I wonder what happens if you put a WARN if cpu_to_node is used
before it is set up?


> @@ -672,10 +672,19 @@ static void __init pcpu_fc_free(void *ptr, size_t size)
>  
>  static int pcpu_cpu_distance(unsigned int from, unsigned int to)
>  {
> -	if (cpu_to_node(from) == cpu_to_node(to))
> -		return LOCAL_DISTANCE;
> -	else
> -		return REMOTE_DISTANCE;
> +#ifndef CONFIG_NUMA
> +	return LOCAL_DISTANCE;
> +#else
> +	int from_nid, to_nid;
> +
> +	from_nid = early_cpu_to_node(from);
> +	to_nid   = early_cpu_to_node(to);
> +
> +	if (from_nid == -1 || to_nid == -1)
> +		return LOCAL_DISTANCE;	/* Or assume remote? */
> +
> +	return node_distance(from_nid, to_nid);

If you made node_distance() return LOCAL_NODE for !NUMA, this
should fall out and not require the ifdef?

Looks good to me though.

Thanks,
Nick