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

[Mark Rutland <mark.rutland@arm.com>]

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()?

From a glance it seems that powerpc does just that.

Mark.

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