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SubjectRe: [PATCH v2 4/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory
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Le 03/09/2019 à 07:23, Alastair D'Silva a écrit :
> From: Alastair D'Silva <alastair@d-silva.org>
>
> When presented with large amounts of memory being hotplugged
> (in my test case, ~890GB), the call to flush_dcache_range takes
> a while (~50 seconds), triggering RCU stalls.
>
> This patch breaks up the call into 1GB chunks, calling
> cond_resched() inbetween to allow the scheduler to run.
>
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
> arch/powerpc/mm/mem.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index cd540123874d..854aaea2c6ae 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -104,11 +104,14 @@ int __weak remove_section_mapping(unsigned long start, unsigned long end)
> return -ENODEV;
> }
>
> +#define FLUSH_CHUNK_SIZE SZ_1G

Maybe the name is a bit long for a local define. See if we could reduce
code line splits below by shortening this name.

> +
> int __ref arch_add_memory(int nid, u64 start, u64 size,
> struct mhp_restrictions *restrictions)
> {
> unsigned long start_pfn = start >> PAGE_SHIFT;
> unsigned long nr_pages = size >> PAGE_SHIFT;
> + u64 i;
> int rc;
>
> resize_hpt_for_hotplug(memblock_phys_mem_size());
> @@ -120,7 +123,12 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
> start, start + size, rc);
> return -EFAULT;
> }
> - flush_dcache_range(start, start + size);
> +
> + for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> + flush_dcache_range(start + i,
> + min(start + size, start + i + FLUSH_CHUNK_SIZE));

My eyes don't like it.

What about
for (; i < size; i += FLUSH_CHUNK_SIZE) {
int len = min(size - i, FLUSH_CHUNK_SIZE);

flush_dcache_range(start + i, start + i + len);
cond_resched();
}

or

end = start + size;
for (; start < end; start += FLUSH_CHUNK_SIZE, size -= FLUSH_CHUNK_SIZE) {
int len = min(size, FLUSH_CHUNK_SIZE);

flush_dcache_range(start, start + len);
cond_resched();
}

> + cond_resched();
> + }
>
> return __add_pages(nid, start_pfn, nr_pages, restrictions);
> }
> @@ -131,13 +139,19 @@ void __ref arch_remove_memory(int nid, u64 start, u64 size,
> unsigned long start_pfn = start >> PAGE_SHIFT;
> unsigned long nr_pages = size >> PAGE_SHIFT;
> struct page *page = pfn_to_page(start_pfn) + vmem_altmap_offset(altmap);
> + u64 i;
> int ret;
>
> __remove_pages(page_zone(page), start_pfn, nr_pages, altmap);
>
> /* Remove htab bolted mappings for this section of memory */
> start = (unsigned long)__va(start);
> - flush_dcache_range(start, start + size);
> + for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> + flush_dcache_range(start + i,
> + min(start + size, start + i + FLUSH_CHUNK_SIZE));
> + cond_resched();
> + }
> +

This piece of code looks pretty similar to the one before. Can we
refactor into a small helper ?

> ret = remove_section_mapping(start, start + size);
> WARN_ON_ONCE(ret);
>
>

Christophe

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