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    SubjectRe: [patch 1/2] x86,pat Update the page flags for memtype atomically instead of using memtype_lock. -V3
    Date
    On Monday 15 March 2010, holt@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@redhat.com>
    > To: H. Peter Anvin <hpa@zytor.com>
    > To: Thomas Gleixner <tglx@linutronix.de>
    > Signed-off-by: Robin Holt <holt@sgi.com>
    > Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    > Cc: Venkatesh Pallipadi <venkatesh.pallipadi@gmail.com>
    > Cc: Suresh Siddha <suresh.b.siddha@intel.com>
    > Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
    > Cc: x86@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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