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Date: Sat, 13 Oct 2018 13:49:06 -0400
From: Pavel Tatashin <pasha.tatashin@...il.com>
To: Alexander Duyck <alexander.duyck@...il.com>
Cc: alexander.h.duyck@...ux.intel.com,
Linux Memory Management List <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Pasha Tatashin <pavel.tatashin@...rosoft.com>,
Michal Hocko <mhocko@...e.com>, dave.jiang@...el.com,
LKML <linux-kernel@...r.kernel.org>, willy@...radead.org,
davem@...emloft.net, yi.z.zhang@...ux.intel.com,
khalid.aziz@...cle.com, rppt@...ux.vnet.ibm.com,
Vlastimil Babka <vbabka@...e.cz>, sparclinux@...r.kernel.org,
dan.j.williams@...el.com, ldufour@...ux.vnet.ibm.com,
mgorman@...hsingularity.net, mingo@...nel.org,
kirill.shutemov@...ux.intel.com
Subject: Re: [mm PATCH v2 1/6] mm: Use mm_zero_struct_page from SPARC on all
64b architectures
Still, lets get some real data, please provide Intel data before vs
after. I could test on an ARM processor.
Pavel
On Sat, Oct 13, 2018 at 1:18 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> Well in the case of x86 the call to memset is expensive as well. In
> most cases it is 16 cycles plus 1 cycle per 16 bytes if I recall
> correctly. So for example in the case of skbuff which was a little
> over 192 bytes I know Jesper Brouer and myself were going back and
> forth with the idea of if we should try to do something similar.
>
> I'm suspecting for the 64b architectures impacted by this change there
> should be little to no negative impact. The main reason for that being
> the fact that the compiler can actually drop some of the writes by
> merging them with the later assignments.
>
> Thanks.
>
> - Alex
>
> On Sat, Oct 13, 2018 at 9:58 AM Pavel Tatashin <pasha.tatashin@...il.com> wrote:
> >
> > I am worried about this change. I added SPARC optimized
> > mm_zero_struct_page() specifically to SPARC because it has a poor
> > performance with small memset()s, since it uses STBI instructions.
> > However, other architectures might not suffer with small memset()s,
> > and have hardware optimized memset variants for small sizes. Don't
> > forget, this is a leaf routine on most arches, so the function call
> > should be cheap. Also, the macro itself is not very flexible: when
> > size of struct page is changed, it also must be modified (we could add
> > fall throughs though), I would add this macro only to those arches
> > that benefit from this change, in other words, I would like to see
> > performance data.
> >
> > I will review the rest of the patches in this series on Monday.
> >
> > Thank you,
> > Pavel
> > On Thu, Oct 11, 2018 at 6:17 PM Alexander Duyck
> > <alexander.h.duyck@...ux.intel.com> wrote:
> > >
> > > This change makes it so that we use the same approach that was already in
> > > use on Sparc on all the archtectures that support a 64b long.
> > >
> > > This is mostly motivated by the fact that 8 to 10 store/move instructions
> > > are likely always going to be faster than having to call into a function
> > > that is not specialized for handling page init.
> > >
> > > An added advantage to doing it this way is that the compiler can get away
> > > with combining writes in the __init_single_page call. As a result the
> > > memset call will be reduced to only about 4 write operations, or at least
> > > that is what I am seeing with GCC 6.2 as the flags, LRU poitners, and
> > > count/mapcount seem to be cancelling out at least 4 of the 8 assignments on
> > > my system.
> > >
> > > One change I had to make to the function was to reduce the minimum page
> > > size to 56 to support some powerpc64 configurations.
> > >
> > > Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> > > ---
> > > arch/sparc/include/asm/pgtable_64.h | 30 ------------------------------
> > > include/linux/mm.h | 34 ++++++++++++++++++++++++++++++++++
> > > 2 files changed, 34 insertions(+), 30 deletions(-)
> > >
> > > diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> > > index 1393a8ac596b..22500c3be7a9 100644
> > > --- a/arch/sparc/include/asm/pgtable_64.h
> > > +++ b/arch/sparc/include/asm/pgtable_64.h
> > > @@ -231,36 +231,6 @@
> > > extern struct page *mem_map_zero;
> > > #define ZERO_PAGE(vaddr) (mem_map_zero)
> > >
> > > -/* This macro must be updated when the size of struct page grows above 80
> > > - * or reduces below 64.
> > > - * The idea that compiler optimizes out switch() statement, and only
> > > - * leaves clrx instructions
> > > - */
> > > -#define mm_zero_struct_page(pp) do { \
> > > - unsigned long *_pp = (void *)(pp); \
> > > - \
> > > - /* Check that struct page is either 64, 72, or 80 bytes */ \
> > > - BUILD_BUG_ON(sizeof(struct page) & 7); \
> > > - BUILD_BUG_ON(sizeof(struct page) < 64); \
> > > - BUILD_BUG_ON(sizeof(struct page) > 80); \
> > > - \
> > > - switch (sizeof(struct page)) { \
> > > - case 80: \
> > > - _pp[9] = 0; /* fallthrough */ \
> > > - case 72: \
> > > - _pp[8] = 0; /* fallthrough */ \
> > > - default: \
> > > - _pp[7] = 0; \
> > > - _pp[6] = 0; \
> > > - _pp[5] = 0; \
> > > - _pp[4] = 0; \
> > > - _pp[3] = 0; \
> > > - _pp[2] = 0; \
> > > - _pp[1] = 0; \
> > > - _pp[0] = 0; \
> > > - } \
> > > -} while (0)
> > > -
> > > /* PFNs are real physical page numbers. However, mem_map only begins to record
> > > * per-page information starting at pfn_base. This is to handle systems where
> > > * the first physical page in the machine is at some huge physical address,
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 273d4dbd3883..dee407998366 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -102,8 +102,42 @@ static inline void set_max_mapnr(unsigned long limit) { }
> > > * zeroing by defining this macro in <asm/pgtable.h>.
> > > */
> > > #ifndef mm_zero_struct_page
> > > +#if BITS_PER_LONG == 64
> > > +/* This function must be updated when the size of struct page grows above 80
> > > + * or reduces below 64. The idea that compiler optimizes out switch()
> > > + * statement, and only leaves move/store instructions
> > > + */
> > > +#define mm_zero_struct_page(pp) __mm_zero_struct_page(pp)
> > > +static inline void __mm_zero_struct_page(struct page *page)
> > > +{
> > > + unsigned long *_pp = (void *)page;
> > > +
> > > + /* Check that struct page is either 56, 64, 72, or 80 bytes */
> > > + BUILD_BUG_ON(sizeof(struct page) & 7);
> > > + BUILD_BUG_ON(sizeof(struct page) < 56);
> > > + BUILD_BUG_ON(sizeof(struct page) > 80);
> > > +
> > > + switch (sizeof(struct page)) {
> > > + case 80:
> > > + _pp[9] = 0; /* fallthrough */
> > > + case 72:
> > > + _pp[8] = 0; /* fallthrough */
> > > + default:
> > > + _pp[7] = 0; /* fallthrough */
> > > + case 56:
> > > + _pp[6] = 0;
> > > + _pp[5] = 0;
> > > + _pp[4] = 0;
> > > + _pp[3] = 0;
> > > + _pp[2] = 0;
> > > + _pp[1] = 0;
> > > + _pp[0] = 0;
> > > + }
> > > +}
> > > +#else
> > > #define mm_zero_struct_page(pp) ((void)memset((pp), 0, sizeof(struct page)))
> > > #endif
> > > +#endif
> > >
> > > /*
> > > * Default maximum number of active map areas, this limits the number of vmas
> > >
> >
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