RE: [PATCH 4/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory

["Alastair D'Silva" <alastair@au1.ibm.com>]

On Thu, 2019-08-15 at 09:36 +0200, christophe leroy wrote:
> 
> Le 15/08/2019 à 06:10, 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 16GB chunks, calling
> > cond_resched() inbetween to allow the scheduler to run.
> 
> Is 16GB small enough ? If 890GB takes 50s, 16GB still takes about 1s.
> I'd use 1GB chuncks to remain below 100ms.
> 
> > Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> > ---
> >   arch/powerpc/mm/mem.c | 16 ++++++++++++++--
> >   1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> > index 5400da87a804..fb0d5e9aa11b 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 (16ull * 1024ull * 1024ull * 1024ull)
> 
> Can we use SZ_16GB ?
Sure, I'll go with 1GB as you recommended above

> > +
> >   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;
> > +	unsigned long i;
> >   	int rc;
> >   
> >   	resize_hpt_for_hotplug(memblock_phys_mem_size());
> > @@ -120,7 +123,11 @@ 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));
> 
> Isn't the line a bit long (I have not checked).
> 
> > +		cond_resched();
> > +	}
> >   
> >   	return __add_pages(nid, start_pfn, nr_pages, restrictions);
> >   }
> > @@ -131,13 +138,18 @@ 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);
> > +	unsigned long 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();
> > +	}
> > +
> >   	ret = remove_section_mapping(start, start + size);
> >   	WARN_ON_ONCE(ret);
> >   
> > 
> 
> Christophe
> 
> ---
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> 
-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819