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From: Rafael J. Wysocki on 23 Mar 2010 19:20
On Monday 15 March 2010, holt(a)sgi.com wrote:
>
> While testing an application using the xpmem (out of kernel) driver, we
> noticed a significant page fault rate reduction of x86_64 with respect
> to ia64. For one test running with 32 cpus, one thread per cpu, it
> took 01:08 for each of the threads to vm_insert_pfn 2GB worth of pages.
> For the same test running on 256 cpus, one thread per cpu, it took 14:48
> to vm_insert_pfn 2 GB worth of pages.
>
> The slowdown was tracked to lookup_memtype which acquires the
> spinlock memtype_lock. This heavily contended lock was slowing down
> vm_insert_pfn().
>
> With the cmpxchg on page->flags method, both the 32 cpu and 256 cpu
> cases take approx 00:01.3 seconds to complete.
>
>
> To: Ingo Molnar <mingo(a)redhat.com>
> To: H. Peter Anvin <hpa(a)zytor.com>
> To: Thomas Gleixner <tglx(a)linutronix.de>
> Signed-off-by: Robin Holt <holt(a)sgi.com>
> Cc: Venkatesh Pallipadi <venkatesh.pallipadi(a)intel.com>
> Cc: Venkatesh Pallipadi <venkatesh.pallipadi(a)gmail.com>
> Cc: Suresh Siddha <suresh.b.siddha(a)intel.com>
> Cc: Linux Kernel Mailing List <linux-kernel(a)vger.kernel.org>
> Cc: x86(a)kernel.org
>
> ---
>
> Changes since -V2:
> 1) Cleared up the naming of the masks used in setting and clearing
> the flags.
>
>
> Changes since -V1:
> 1) Introduce atomically setting and clearing the page flags and not
> using the global memtype_lock to protect page->flags.
>
> 2) This allowed me the opportunity to convert the rwlock back into a
> spinlock and not affect _MY_ tests performance as all the pages my test
> was utilizing are tracked by struct pages.
>
> 3) Corrected the commit log. The timings were for 32 cpus and not 256.
>
> arch/x86/include/asm/cacheflush.h | 44 +++++++++++++++++++++-----------------
> arch/x86/mm/pat.c | 8 ------
> 2 files changed, 25 insertions(+), 27 deletions(-)
>
> Index: linux-next/arch/x86/include/asm/cacheflush.h
> ===================================================================
> --- linux-next.orig/arch/x86/include/asm/cacheflush.h 2010-03-12 19:55:06.690471974 -0600
> +++ linux-next/arch/x86/include/asm/cacheflush.h 2010-03-12 19:55:41.846472324 -0600
> @@ -44,9 +44,6 @@ static inline void copy_from_user_page(s
> memcpy(dst, src, len);
> }
>
> -#define PG_WC PG_arch_1
> -PAGEFLAG(WC, WC)
> -
> #ifdef CONFIG_X86_PAT
> /*
> * X86 PAT uses page flags WC and Uncached together to keep track of
> @@ -55,16 +52,24 @@ PAGEFLAG(WC, WC)
> * _PAGE_CACHE_UC_MINUS and fourth state where page's memory type has not
> * been changed from its default (value of -1 used to denote this).
> * Note we do not support _PAGE_CACHE_UC here.
> - *
> - * Caller must hold memtype_lock for atomicity.
> */
> +
> +#define _PGMT_DEFAULT 0
> +#define _PGMT_WC PG_arch_1
> +#define _PGMT_UC_MINUS PG_uncached
> +#define _PGMT_WB (PG_uncached | PG_arch_1)
> +#define _PGMT_MASK (PG_uncached | PG_arch_1)
> +#define _PGMT_CLEAR_MASK (~_PGMT_MASK)
> +

Can we manipulate the PG_* constants this way? They are just bit numbers,
so for example _PGMT_WB should be ((1 << PG_uncached) | (PG_arch_1)) for
example.

Rafael
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