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    SubjectRe: [profile]: [0/23] mmap() support for /proc/profile

    > I was trying to profile a mostly-idle workload to get an idea of what
    > area of the kernel things were diving into and falling asleep in during
    > an OAST run. Without these patches, kerneltop et al showed heavy /proc/
    > activity along with copy_to_user() at the top of the profiles.

    Interesting stuff. FYI we did some analysis of the hottest addresses in
    the kernel and profile_lock featured very high up:

    void profile_hook(struct pt_regs * regs)
    {
    read_lock(&profile_lock);
    notifier_call_chain(&profile_listeners, 0, regs);
    read_unlock(&profile_lock);
    }

    Thats 2 atomic operations to the same cacheline per timer interrupt per
    cpu. Considering how rarely timer based profiling is used, perhaps RCU
    or even just a profiling_enabled sysctl flag would help here. Id prefer
    not to compile it out in distro kernels if possible, its a very useful
    feature when required.

    In the mean time, how about this quick fix?

    Anton

    --

    Cacheline align profile_lock, analysis shows it to be one of the hottest
    memory locations on large SMP boxes.

    Signed-off-by: Anton Blanchard <anton@samba.org>

    ===== kernel/profile.c 1.5 vs edited =====
    --- 1.5/kernel/profile.c Wed Jul 16 18:09:04 2003
    +++ edited/kernel/profile.c Wed Jun 23 09:13:28 2004
    @@ -8,6 +8,7 @@
    #include <linux/bootmem.h>
    #include <linux/notifier.h>
    #include <linux/mm.h>
    +#include <linux/cache.h>
    #include <asm/sections.h>

    unsigned int * prof_buffer;
    @@ -119,7 +120,7 @@
    }

    static struct notifier_block * profile_listeners;
    -static rwlock_t profile_lock = RW_LOCK_UNLOCKED;
    +static rwlock_t profile_lock __cacheline_aligned_in_smp = RW_LOCK_UNLOCKED;

    int register_profile_notifier(struct notifier_block * nb)
    {
    -
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