Re: [PATCH 4.4 66/90] powerpc/numa: Fix percpu allocations to be NUMA aware

[Michal Hocko]

JFYI. We have encountered a regression after applying this patch on a
large ppc machine. While the patch is the right thing to do it doesn't
work well with the current vmalloc area size on ppc and large machines
where NUMA nodes are very far from each other. Just for the reference
the boot fails on such a machine with bunch of warning preceeding it.
See http://lkml.kernel.org/r/20170724134240.GL25221@xxxxxxxxxxxxxx

It seems the right thing to do is to enlarge the vmalloc space on ppc
but this is not the case in the upstream kernel yet AFAIK. It is also
questionable whether that is a stable material but I will decision on
you here.

We have reverted this patch from our 4.4 based kernel.

On Mon 12-06-17 17:26:12, Greg KH wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> 
> commit ba4a648f12f4cd0a8003dd229b6ca8a53348ee4b upstream.
> 
> 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]
> 
> Fixes: 8c272261194d ("powerpc/numa: Enable USE_PERCPU_NUMA_NODE_ID")
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Reviewed-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> ---
>  arch/powerpc/include/asm/topology.h |   14 ++++++++++++++
>  arch/powerpc/kernel/setup_64.c      |    4 ++--
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> --- a/arch/powerpc/include/asm/topology.h
> +++ b/arch/powerpc/include/asm/topology.h
> @@ -44,8 +44,22 @@ extern void __init dump_numa_cpu_topolog
>  extern int sysfs_add_device_to_node(struct device *dev, int nid);
>  extern void sysfs_remove_device_from_node(struct device *dev, int nid);
>  
> +static inline int early_cpu_to_node(int cpu)
> +{
> +	int nid;
> +
> +	nid = numa_cpu_lookup_table[cpu];
> +
> +	/*
> +	 * Fall back to node 0 if nid is unset (it should be, except bugs).
> +	 * This allows callers to safely do NODE_DATA(early_cpu_to_node(cpu)).
> +	 */
> +	return (nid < 0) ? 0 : nid;
> +}
>  #else
>  
> +static inline int early_cpu_to_node(int cpu) { return 0; }
> +
>  static inline void dump_numa_cpu_topology(void) {}
>  
>  static inline int sysfs_add_device_to_node(struct device *dev, int nid)
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -751,7 +751,7 @@ void __init setup_arch(char **cmdline_p)
>  
>  static void * __init pcpu_fc_alloc(unsigned int cpu, size_t size, size_t align)
>  {
> -	return __alloc_bootmem_node(NODE_DATA(cpu_to_node(cpu)), size, align,
> +	return __alloc_bootmem_node(NODE_DATA(early_cpu_to_node(cpu)), size, align,
>  				    __pa(MAX_DMA_ADDRESS));
>  }
>  
> @@ -762,7 +762,7 @@ static void __init pcpu_fc_free(void *pt
>  
>  static int pcpu_cpu_distance(unsigned int from, unsigned int to)
>  {
> -	if (cpu_to_node(from) == cpu_to_node(to))
> +	if (early_cpu_to_node(from) == early_cpu_to_node(to))
>  		return LOCAL_DISTANCE;
>  	else
>  		return REMOTE_DISTANCE;
> 

-- 
Michal Hocko
SUSE Labs