Subject: Re: [PATCH] arm64: numa: rightsize the distance array

From: Jonathan Cameron <[email-masked]>
Date: Thu, 20 Aug 2020 09:45:15 +0100

On Thu, 20 Aug 2020 08:39:14 +0530
Anshuman Khandual <[email-masked]> wrote:

> On 08/19/2020 03:11 PM, Jonathan Cameron wrote:
> > On Wed, 19 Aug 2020 10:31:18 +0530
> > Anshuman Khandual <[email-masked]> wrote:
> >   
> >> Hello Jonathan,  
> > 
> > Hi Anshuman,
> >   
> >>
> >> On 07/08/2020 05:08 PM, Jonathan Cameron wrote:  
> >>> Unfortunately we are currently calling numa_alloc_distance well before we call
> >>> setup_node_to_cpu_mask_map means that nr_node_ids is set to MAX_NUMNODES.
> >>> This wastes a bit of memory and is confusing to the reader.    
> >>
> >> With defconfig where CONFIG_NODES_SHIFT = 2 i.e MAX_NUMNODES = 4, the total
> >> numa_distance size is 16 bytes (individual entries here are just u8).   
> > 
> > That's another issue to tidy up.  The current defconfig isn't big enough to
> > support existing platforms (we are shipping 8 node systems),
> > let alone next generation.  Bit of an open question on how far to push it up
> > though. Perhaps 64?  Given CXL we are probably going to soon see systems with
> > a lot more nodes.  I'll send a patch for that and we can argue the exact
> > value then.  
> 
> I would expect atleast it matches up with the max nodes possible on currently
> available systems.
> 
> >   
> >> Even
> >> with MAX_NUMNODES = 256, numa_distance is going to be just 64K. Hence there
> >> might not be much space to be saved that would need optimizing this path.
> >> Please correct me if I have missed something.  
> > 
> > Agreed.  The saving is small.  This was mostly about the code being misleading.
> > It gives the impression of doing dynamic sizing but doesn't actually do so as it
> > is using nr_node_ids before that value has been updated.  I would prefer
> > to make the code 'obviously' correct.  The x86 path is one approach, simply
> > hard coding the size as a constant is another.
> > 
> > The secondary advantage would be to make it easy to unify this code across
> > architectures.  Kind of inevitable riscv will be here soon and it would be nice
> > not to have a third slight variation on the code.  I'd like to explore such
> > possible unification but it wasn't the main aim of this patch.  
> 
> There is already a RISC-V proposal in this regard which is trying to create
> a generic NUMA framework selectable with GENERIC_ARCH_NUMA. If the ultimate
> goal is unify the NUMA init path, we should try and see if that can also
> accommodate X86.
> 
> https://patchwork.kernel.org/project/linux-arm-kernel/list/?series=332897

Agreed.

It may be the case that we need to do that in a number of steps but it is definitely
a good place to aim for.

Perhaps we can ignore ia64 however :)

Jonathan

> 
> >   
> >>  
> >>>
> >>> Note we could just decide to hardcode it as MAX_NUMNODES but if so we should
> >>> do so explicitly.    
> >>
> >> nr_node_ids = MAX_NUMNODES which is set from mm/page_alloc.c, yes asserting
> >> with an WARN_ON() that it is indeed MAX_NUMNODES would make sense.  
> > 
> > OK, a WARN_ON would at least make it apparent this is a constant, not a dynamic
> > value as a reader of the code might assume (I did).
> > 
> > Could we just go further and use MAX_NUMNODES directly and ignore nr_node_ids
> > for this purpose?  
> 
> IMHO this actually sounds better and cleaner. But the first preference should
> be to unify all these as GENERIC_ARCH_NUMA.
> 
> >   
> >>  
> >>>
> >>> Looking at what x86 does, they do a walk of nodes_parsed and locally
> >>> establish the maximum node count seen.  We can't actually do that where we
> >>> were previously calling it in numa_init because nodes_parsed isn't set up
> >>> either yet.  So let us take a leaf entirely out of x86's book and make
> >>> the true assumption that nodes_parsed will definitely be set up before
> >>> we try to put a real value in this array.  Hence just do it on demand.    
> >>
> >> So it is replacing one assumption i.e nr_node_ids = MAX_NUMNODES with another
> >> i.e nodes_parsed has been initialized, while trying to populate an entry.  
> > 
> > Yes.  If we make it clearly a fixed value, then I'm happy with that solution
> > to this particular problem.
> > 
> > If not, I would argue that the use nodes_parsed makes it very much obvious that
> > we are assuming it was initialized.  (Though on that argument we would have
> > assumed nr_node_ids had it's final value in the existing code and it doesn't,
> > I come back to my main argument being to make the code 'obviously' correct).
> >   
> >>  
> >>>
> >>> In order to avoid trying and failing to allocate the array multiple times
> >>> we do the same thing as x86 and set numa_distance = 1. This requires a
> >>> few small modifications elsewhere.    
> >>
> >> Where ? Dont see numa_distance being set as 1.  
> > 
> > See below, I have highlighted the line.
> >   
> >>  
> >>>
> >>> Worth noting, that with one exception (which it appears can be removed [1])
> >>> the x86 and arm numa distance code is now identical.  Worth factoring it
> >>> out to some common location?
> >>>
> >>> [1] https://lkml.kernel.org/r/[email-masked]
> >>>
> >>> Signed-off-by: Jonathan Cameron <[email-masked]>
> >>> ---
> >>> arch/arm64/mm/numa.c | 35 ++++++++++++++++++-----------------
> >>>  1 file changed, 18 insertions(+), 17 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/mm/numa.c b/arch/arm64/mm/numa.c
> >>> index aafcee3e3f7e..a2f549ef0a36 100644
> >>> --- a/arch/arm64/mm/numa.c
> >>> +++ b/arch/arm64/mm/numa.c
> >>> @@ -255,13 +255,11 @@ void __init numa_free_distance(void)
> >>>  {
> >>>  	size_t size;
> >>>  
> >>> -	if (!numa_distance)
> >>> -		return;
> >>> -
> >>>  	size = numa_distance_cnt * numa_distance_cnt *
> >>>  		sizeof(numa_distance[0]);
> >>> -
> >>> -	memblock_free(__pa(numa_distance), size);
> >>> +	/* numa_distance could be 1LU marking allocation failure, test cnt */
> >>> +	if (numa_distance_cnt)
> >>> +		memblock_free(__pa(numa_distance), size);
> >>>  	numa_distance_cnt = 0;
> >>>  	numa_distance = NULL;
> >>>  }
> >>> @@ -271,20 +269,29 @@ void __init numa_free_distance(void)
> >>>   */
> >>>  static int __init numa_alloc_distance(void)
> >>>  {
> >>> +	nodemask_t nodes_parsed;
> >>>  	size_t size;
> >>> +	int i, j, cnt = 0;
> >>>  	u64 phys;
> >>> -	int i, j;
> >>>  
> >>> -	size = nr_node_ids * nr_node_ids * sizeof(numa_distance[0]);
> >>> +	/* size the new table and allocate it */
> >>> +	nodes_parsed = numa_nodes_parsed;
> >>> +	for_each_node_mask(i, nodes_parsed)
> >>> +		cnt = i;    
> >>
> >> There is no nodemask related helper to fetch the highest bit set ?  
> > 
> > Not that I can find.
> > 
> > There is nodes_weight(), but I think we can currently end up with holes,
> > and it definitely isn't obvious from the local code that we can't.  
> 
> Right, the node mask can have holes, so nodes_weight() would not imply
> maximum possible node number. Possibly a new helper nodes_max() should
> do but not sure if there are any existing instances which could use
> such a helper.  
> 
> > 
> >   
> >>  
> >>> +	cnt++;
> >>> +	size = cnt * cnt * sizeof(numa_distance[0]);
> >>>  	phys = memblock_find_in_range(0, PFN_PHYS(max_pfn),
> >>>  				      size, PAGE_SIZE);
> >>> -	if (WARN_ON(!phys))
> >>> +	if (!phys) {
> >>> +		pr_warn("Warning: can't allocate distance table!\n");
> >>> +		/* don't retry until explicitly reset */
> >>> +		numa_distance = (void *)1LU;  
> > 
> > Here is where we set numa_distance to 1 in the error path.  
> 
> Got it.
> 
> >   
> >>>  		return -ENOMEM;
> >>> -
> >>> +	}
> >>>  	memblock_reserve(phys, size);
> >>>  
> >>>  	numa_distance = __va(phys);
> >>> -	numa_distance_cnt = nr_node_ids;
> >>> +	numa_distance_cnt = cnt;
> >>>  
> >>>  	/* fill with the default distances */
> >>>  	for (i = 0; i < numa_distance_cnt; i++)
> >>> @@ -311,10 +318,8 @@ static int __init numa_alloc_distance(void)
> >>>   */
> >>>  void __init numa_set_distance(int from, int to, int distance)
> >>>  {
> >>> -	if (!numa_distance) {
> >>> -		pr_warn_once("Warning: distance table not allocated yet\n");
> >>> +	if (!numa_distance && numa_alloc_distance() < 0)
> >>>  		return;
> >>> -	}
> >>>  
> >>>  	if (from >= numa_distance_cnt || to >= numa_distance_cnt ||
> >>>  			from < 0 || to < 0) {
> >>> @@ -384,10 +389,6 @@ static int __init numa_init(int (*init_func)(void))
> >>>  	nodes_clear(node_possible_map);
> >>>  	nodes_clear(node_online_map);
> >>>  
> >>> -	ret = numa_alloc_distance();
> >>> -	if (ret < 0)
> >>> -		return ret;
> >>> -
> >>>  	ret = init_func();
> >>>  	if (ret < 0)
> >>>  		goto out_free_distance;
> >>>     
> >>
> >> What is the primary objective here ? Reduce memory for numa_distance[]
> >> or unifying arm64's numa_init() with that of x86's ?  
> > 
> > As mentioned above. The objective when I originally looked at this was
> > to make the code do what it appeared to do.  The approach chosen was about
> > the added benefit of unifying this part with x86 + riscv etc.
> > Always good to not reinvent the wheel.  
> 
> Agreed.

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