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SubjectRe: [PATCH v2 4/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory
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On Tue, 2019-09-03 at 08:19 +0200, Christophe Leroy wrote:
>
> 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 ?
>

Not much point, it's removed in a subsequent patch.

--
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819

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