Re: [PATCH 2/2] mm: hugetlb: Use node interface of cma

[Michal Hocko]

On Thu 02-04-20 17:20:01, Vlastimil Babka wrote:
[...]
> FWIW, for review purposes, this is Roman's patch with all followups from
> mmotm/next (hopefully didn't miss any) and then squashed with patch 2/2 from
> this thread. It can be applied like this:
> 
> - checkout v5.6
> - apply patch 1/2 from this thread
> - apply below

Thanks!

> ----8<----
> >From dc10a593f2b8dfc7be920b4b088a8d55068fc6bc Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@xxxxxx>
> Date: Thu, 2 Apr 2020 13:49:04 +1100
> Subject: [PATCH] mm: hugetlb: optionally allocate gigantic hugepages using cma
> 
> Commit 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at
> runtime") has added the run-time allocation of gigantic pages.  However it
> actually works only at early stages of the system loading, when the
> majority of memory is free.  After some time the memory gets fragmented by
> non-movable pages, so the chances to find a contiguous 1 GB block are
> getting close to zero.  Even dropping caches manually doesn't help a lot.
> 
> At large scale rebooting servers in order to allocate gigantic hugepages
> is quite expensive and complex.  At the same time keeping some constant
> percentage of memory in reserved hugepages even if the workload isn't
> using it is a big waste: not all workloads can benefit from using 1 GB
> pages.
> 
> The following solution can solve the problem:
> 1) On boot time a dedicated cma area* is reserved. The size is passed
>    as a kernel argument.
> 2) Run-time allocations of gigantic hugepages are performed using the
>    cma allocator and the dedicated cma area
> 
> In this case gigantic hugepages can be allocated successfully with a high
> probability, however the memory isn't completely wasted if nobody is using
> 1GB hugepages: it can be used for pagecache, anon memory, THPs, etc.
> 
> * On a multi-node machine a per-node cma area is allocated on each node.
>   Following gigantic hugetlb allocation are using the first available
>   numa node if the mask isn't specified by a user.
> 
> Usage:
> 1) configure the kernel to allocate a cma area for hugetlb allocations:
>    pass hugetlb_cma=10G as a kernel argument
> 
> 2) allocate hugetlb pages as usual, e.g.
>    echo 10 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> 
> If the option isn't enabled or the allocation of the cma area failed,
> the current behavior of the system is preserved.
> 
> x86 and arm-64 are covered by this patch, other architectures can be
> trivially added later.
> 
> Link: http://lkml.kernel.org/r/20200311220920.2487528-1-guro@xxxxxx
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Tested-by: Andreas Schaufler <andreas.schaufler@xxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: Andreas Schaufler <andreas.schaufler@xxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Joonsoo Kim <js1304@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> 
> mm: hugetlb: Use node interface of cma
> 
> With introduction of numa node interface for CMA, this patch is for using that
> interface for allocating memory on numa nodes if NUMA is configured.
> This will be more efficient  and cleaner because first, instead of iterating
> mem range of each numa node, cma_declare_contigueous_nid() will do
> its own address finding if we pass 0 for  both min_pfn and max_pfn,
> second, it can also handle caseswhere NUMA is not configured
> by passing NUMA_NO_NODE as an argument.
> 
> In addition, checking if desired size of memory is available or not,
> is happening in cma_declare_contiguous_nid()  because base and
> limit will be determined there, since 0(any) for  base and
> 0(any) for limit is passed as argument to the function.
> 
> Signed-off-by: Aslan Bakirov <aslan@xxxxxx>
> Acked-by: Roman Gushchin <guro@xxxxxx>

Minor nit below. For the squashed version feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  .../admin-guide/kernel-parameters.txt         |  7 ++
>  arch/arm64/mm/init.c                          |  6 ++
>  arch/x86/kernel/setup.c                       |  4 +
>  include/linux/hugetlb.h                       |  8 ++
>  mm/hugetlb.c                                  | 98 +++++++++++++++++++
>  5 files changed, 123 insertions(+)
> 

[...]

> +	reserved = 0;
> +	for_each_node_state(nid, N_ONLINE) {
> +		int res;
> +
> +		size = min(per_node, hugetlb_cma_size - reserved);
> +		size = round_up(size, PAGE_SIZE << order);
> +		
> +		
> +#ifndef CONFIG_NUMA
> +		nid = NUMA_NO_NODE
> +#endif		

This can be dropped. UMA will simply use node 0 and the memblock
allocator will just do the right thing.

> +		res = cma_declare_contiguous_nid(0, size,
> +					     0, 
> +					     PAGE_SIZE << order,
> +					     0, false,
> +					     "hugetlb", &hugetlb_cma[nid], nid);		
> +
> +		if (res) {
> +			pr_warn("%s: reservation failed: err %d, node %d\n",
> +				__func__, res, nid);
> +			break;
> +		}
> +
> +		reserved += size;
> +		pr_info("hugetlb_cma: reserved %lu MiB on node %d\n",
> +			size / SZ_1M, nid);
> +
> +		if (reserved >= hugetlb_cma_size)
> +			break;
> +	}
> +}
> +
> +#endif /* CONFIG_CMA */
> -- 
> 2.26.0

-- 
Michal Hocko
SUSE Labs