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

From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Steve Capper <steve.capper@linaro.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	"linux@arm.linux.org.uk" <linux@arm.linux.org.uk>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"anders.roxell@linaro.org" <anders.roxell@linaro.org>,
	"gary.robertson@linaro.org" <gary.robertson@linaro.org>,
	Will Deacon <Will.Deacon@arm.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"christoffer.dall@linaro.org" <christoffer.dall@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 6/6] arm64: mm: Enable RCU fast_gup
Date: Thu, 26 Jun 2014 09:56:05 +0200	[thread overview]
Message-ID: <20140626075605.GB12054@laptop.lan> (raw)
In-Reply-To: <20140625165003.GI15240@leverpostej>

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.

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