From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH V4 0/2] mm: FAULT_AROUND_ORDER patchset performance data for powerpc Date: Tue, 20 May 2014 12:59:56 -0700 Message-ID: <20140520125956.aa61a3bfd84d4d6190740ce2@linux-foundation.org> References: <1399541296-18810-1-git-send-email-maddy@linux.vnet.ibm.com> <537479E7.90806@linux.vnet.ibm.com> <87wqdik4n5.fsf@rustcorp.com.au> <53797511.1050409@linux.vnet.ibm.com> <20140519164301.eafd3dd288ccb88361ddcfc7@linux-foundation.org> <20140520004429.E660AE009B@blue.fi.intel.com> <87oaythsvk.fsf@rustcorp.com.au> <20140520102738.7F096E009B@blue.fi.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: Received: from mail.linuxfoundation.org ([140.211.169.12]:34266 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752647AbaETT75 (ORCPT ); Tue, 20 May 2014 15:59:57 -0400 In-Reply-To: <20140520102738.7F096E009B@blue.fi.intel.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Kirill A. Shutemov" Cc: Rusty Russell , Hugh Dickins , Madhavan Srinivasan , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, x86@kernel.org, benh@kernel.crashing.org, paulus@samba.org, riel@redhat.com, mgorman@suse.de, ak@linux.intel.com, peterz@infradead.org, mingo@kernel.org, dave.hansen@intel.com On Tue, 20 May 2014 13:27:38 +0300 (EEST) "Kirill A. Shutemov" wrote: > Rusty Russell wrote: > > "Kirill A. Shutemov" writes: > > > Andrew Morton wrote: > > >> On Mon, 19 May 2014 16:23:07 -0700 (PDT) Hugh Dickins wrote: > > >> > > >> > Shouldn't FAULT_AROUND_ORDER and fault_around_order be changed to be > > >> > the order of the fault-around size in bytes, and fault_around_pages() > > >> > use 1UL << (fault_around_order - PAGE_SHIFT) > > >> > > >> Yes. And shame on me for missing it (this time!) at review. > > >> > > >> There's still time to fix this. Patches, please. > > > > > > Here it is. Made at 3.30 AM, build tested only. > > > > Prefer on top of Maddy's patch which makes it always a variable, rather > > than CONFIG_DEBUG_FS. It's got enough hair as it is. > > Something like this? This appears to be against mainline, not against Madhavan's patch. As mentioned previously, I'd prefer it that way but confused. > From: "Kirill A. Shutemov" > Date: Tue, 20 May 2014 13:02:03 +0300 > Subject: [PATCH] mm: nominate faultaround area in bytes rather then page order > > There are evidences that faultaround feature is less relevant on > architectures with page size bigger then 4k. Which makes sense since > page fault overhead per byte of mapped area should be less there. > > Let's rework the feature to specify faultaround area in bytes instead of > page order. It's 64 kilobytes for now. > > The patch effectively disables faultaround on architectures with > page size >= 64k (like ppc64). > > It's possible that some other size of faultaround area is relevant for a > platform. We can expose `fault_around_bytes' variable to arch-specific > code once such platforms will be found. > > Signed-off-by: Kirill A. Shutemov > --- > mm/memory.c | 62 +++++++++++++++++++++++-------------------------------------- > 1 file changed, 23 insertions(+), 39 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 037b812a9531..252b319e8cdf 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3402,63 +3402,47 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address, > update_mmu_cache(vma, address, pte); > } > > -#define FAULT_AROUND_ORDER 4 > +static unsigned long fault_around_bytes = 65536; > + > +static inline unsigned long fault_around_pages(void) > +{ > + return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE; > +} I think we should round up, not down. So if the user asks for 1kb, they get one page. So this becomes return PAGE_ALIGN(fault_around_bytes) / PAGE_SIZE; > +static inline unsigned long fault_around_mask(void) > +{ > + return ~(rounddown_pow_of_two(fault_around_bytes) - 1) & PAGE_MASK; > +} And this has me a bit stumped. It's not helpful that do_fault_around() is undocumented. Does it fault in N/2 pages ahead and N/2 pages behind? Or does it align the address down to the highest multiple of fault_around_bytes? It appears to be the latter, so the location of the faultaround window around the fault address is basically random, depending on what address userspace happened to pick. I don't know why we did this :( Or something. Can we please get some code commentary over do_fault_around() describing this design decision and explaining the reasoning behind it? Also, "neast" is not a word.