Re: [PATCH -tip 4/5] kprobes/x86: Use text_poke_smp_batch

[Masami Hiramatsu <mhiramat at redhat dot com>]

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.

Thank you,

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