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    SubjectRe: [PATCH -tip 4/5] kprobes/x86: Use text_poke_smp_batch
    * Masami Hiramatsu (mhiramat@redhat.com) wrote:
    > Mathieu Desnoyers wrote:
    > > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
    > >> Use text_poke_smp_batch() in optimization path for reducing
    > >> the number of stop_machine() issues.
    > >>
    > >> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
    > >> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
    > >> Cc: Ingo Molnar <mingo@elte.hu>
    > >> Cc: Jim Keniston <jkenisto@us.ibm.com>
    > >> Cc: Jason Baron <jbaron@redhat.com>
    > >> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    > >> ---
    > >>
    > >> arch/x86/kernel/kprobes.c | 37 ++++++++++++++++++++++++++++++-------
    > >> include/linux/kprobes.h | 2 +-
    > >> kernel/kprobes.c | 13 +------------
    > >> 3 files changed, 32 insertions(+), 20 deletions(-)
    > >>
    > >> diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
    > >> index 345a4b1..63a5c24 100644
    > >> --- a/arch/x86/kernel/kprobes.c
    > >> +++ b/arch/x86/kernel/kprobes.c
    > >> @@ -1385,10 +1385,14 @@ int __kprobes arch_prepare_optimized_kprobe(struct optimized_kprobe *op)
    > >> return 0;
    > >> }
    > >>
    > >> -/* Replace a breakpoint (int3) with a relative jump. */
    > >> -int __kprobes arch_optimize_kprobe(struct optimized_kprobe *op)
    > >> +#define MAX_OPTIMIZE_PROBES 256
    > >
    > > So what kind of interrupt latency does a 256-probes batch generate on the
    > > system ? Are we talking about a few milliseconds, a few seconds ?
    >
    > From my experiment on kvm/4cpu, it took about 3 seconds in average.

    That's 3 seconds for multiple calls to stop_machine(). So we can expect
    latencies in the area of few microseconds for each call, right ?

    > With this patch, it went down to 30ms. (x100 faster :))

    This is beefing up the latency from few microseconds to 30ms. It sounds like a
    regression rather than a gain to me.

    Thanks,

    Mathieu

    >
    > Thank you,
    > --
    > Masami Hiramatsu
    > e-mail: mhiramat@redhat.com
    >

    --
    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
    http://www.efficios.com


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