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    SubjectRe: [PATCH -tip 4/5] kprobes/x86: Use text_poke_smp_batch
    Mathieu Desnoyers wrote:
    > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
    >> Mathieu Desnoyers wrote:
    >>> * 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 ?
    >>
    >> Theoretically yes.
    >> But if we register more than 1000 probes at once, it's hard to do
    >> anything except optimizing a while(more than 10 sec), because
    >> it stops machine so frequently.
    >>
    >>>> 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.
    >>
    >> If it is not acceptable, I can add a knob for control how many probes
    >> optimize/unoptimize at once. Anyway, it is expectable latency (after
    >> registering/unregistering probes) and it will be small if we put a few probes.
    >> (30ms is the worst case)
    >> And if you want, it can be disabled by sysctl.
    >
    > I think we are starting to see the stop_machine() approach is really limiting
    > our ability to do even relatively small amount of work without hurting
    > responsiveness significantly.
    >
    > What's the current showstopper with the breakpoint-bypass-ipi approach that
    > solves this issue properly and makes this batching approach unnecessary ?

    We still do not have any official answer from chip vendors.
    As you know, basic implementation has been done.

    Thank you,

    --
    Masami Hiramatsu
    e-mail: mhiramat@redhat.com


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