Re: [PATCH 6/6] arm64: mm: Enable RCU fast_gup

[Will Deacon <will.deacon@arm.com>]

On Thu, Jun 26, 2014 at 08:56:05AM +0100, Peter Zijlstra wrote:
> On Wed, Jun 25, 2014 at 05:50:03PM +0100, Mark Rutland wrote:
> > Hi Steve,
> > 
> > On Wed, Jun 25, 2014 at 04:40:24PM +0100, Steve Capper wrote:
> > > Activate the RCU fast_gup for ARM64. We also need to force THP splits
> > > to broadcast an IPI s.t. we block in the fast_gup page walker. As THP
> > > splits are comparatively rare, this should not lead to a noticeable
> > > performance degradation.
> > > 
> > > Some pre-requisite functions pud_write and pud_page are also added.
> > > 
> > > Signed-off-by: Steve Capper <steve.capper@linaro.org>
> > > ---
> > >  arch/arm64/Kconfig               |  3 +++
> > >  arch/arm64/include/asm/pgtable.h | 11 ++++++++++-
> > >  arch/arm64/mm/flush.c            | 19 +++++++++++++++++++
> > >  3 files changed, 32 insertions(+), 1 deletion(-)
> > 
> > [...]
> > 
> > > diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
> > > index e4193e3..ddf96c1 100644
> > > --- a/arch/arm64/mm/flush.c
> > > +++ b/arch/arm64/mm/flush.c
> > > @@ -103,3 +103,22 @@ EXPORT_SYMBOL(flush_dcache_page);
> > >   */
> > >  EXPORT_SYMBOL(flush_cache_all);
> > >  EXPORT_SYMBOL(flush_icache_range);
> > > +
> > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > +#ifdef CONFIG_HAVE_RCU_TABLE_FREE
> > > +static void thp_splitting_flush_sync(void *arg)
> > > +{
> > > +}
> > > +
> > > +void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
> > > +			  pmd_t *pmdp)
> > > +{
> > > +	pmd_t pmd = pmd_mksplitting(*pmdp);
> > > +	VM_BUG_ON(address & ~PMD_MASK);
> > > +	set_pmd_at(vma->vm_mm, address, pmdp, pmd);
> > > +
> > > +	/* dummy IPI to serialise against fast_gup */
> > > +	smp_call_function(thp_splitting_flush_sync, NULL, 1);
> > 
> > Is there some reason we can't use kick_all_cpus_sync()?
> 
> Yes that would be equivalent. But looking at that, I worry about the
> smp_mb(); archs are supposed to make sure IPIs are serializing.

Agreed; smp_call_function would be hopelessly broken if that wasn't true
(at least, everywhere I've used it ;)

Will

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