RE: [PATCH v4 5/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory

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

On Fri, 2019-09-27 at 08:37 +0200, Mark Marshall wrote:
> Comment below...
> 
> On Thu, 26 Sep 2019 at 12:18, Alastair D'Silva <alastair@au1.ibm.com>
> wrote:
> > 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 | 27 +++++++++++++++++++++++++--
> >  1 file changed, 25 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> > index cff947cb2a84..a2758e473d58 100644
> > --- a/arch/powerpc/mm/mem.c
> > +++ b/arch/powerpc/mm/mem.c
> > @@ -104,6 +104,27 @@ int __weak remove_section_mapping(unsigned
> > long start, unsigned long end)
> >         return -ENODEV;
> >  }
> > 
> > +#define FLUSH_CHUNK_SIZE SZ_1G
> > +/**
> > + * flush_dcache_range_chunked(): Write any modified data cache
> > blocks out to
> > + * memory and invalidate them, in chunks of up to FLUSH_CHUNK_SIZE
> > + * Does not invalidate the corresponding instruction cache blocks.
> > + *
> > + * @start: the start address
> > + * @stop: the stop address (exclusive)
> > + * @chunk: the max size of the chunks
> > + */
> > +static void flush_dcache_range_chunked(unsigned long start,
> > unsigned long stop,
> > +                                      unsigned long chunk)
> > +{
> > +       unsigned long i;
> > +
> > +       for (i = start; i < stop; i += FLUSH_CHUNK_SIZE) {
> Here you ignore the function parameter "chunk" and use the define
> FLUSH_CHUNK_SIZE.
> You should do one or the other; use the parameter or remove it.

Good catch, thankyou :)

> > +               flush_dcache_range(i, min(stop, start +
> > FLUSH_CHUNK_SIZE));
> > +               cond_resched();
> > +       }
> > +}
> > +
> >  int __ref arch_add_memory(int nid, u64 start, u64 size,
> >                         struct mhp_restrictions *restrictions)
> >  {
> > @@ -120,7 +141,8 @@ int __ref arch_add_memory(int nid, u64 start,
> > u64 size,
> >                         start, start + size, rc);
> >                 return -EFAULT;
> >         }
> > -       flush_dcache_range(start, start + size);
> > +
> > +       flush_dcache_range_chunked(start, start + size,
> > FLUSH_CHUNK_SIZE);
> > 
> >         return __add_pages(nid, start_pfn, nr_pages, restrictions);
> >  }
> > @@ -137,7 +159,8 @@ void __ref arch_remove_memory(int nid, u64
> > start, u64 size,
> > 
> >         /* Remove htab bolted mappings for this section of memory
> > */
> >         start = (unsigned long)__va(start);
> > -       flush_dcache_range(start, start + size);
> > +       flush_dcache_range_chunked(start, start + size,
> > FLUSH_CHUNK_SIZE);
> > +
> >         ret = remove_section_mapping(start, start + size);
> >         WARN_ON_ONCE(ret);
> > 
> > --
> > 2.21.0
> > 
-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819