Re: [PATCH -tip v9 25/26] kprobes: Introduce kprobe cache to reduce cache misshits

Masami Hiramatsu
2014-04-24T11:38:28Z

(2014/04/24 18:01), Ingo Molnar wrote:
>
> * Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
>
>> Introduce kprobe cache to reduce cache misshits for
>> massive multiple kprobes.
>> For stress testing kprobes, we need to activate kprobes
>> as many as possible. This situation causes cache miss
>> hit storm on kprobe hash-list. kprobe hashlist is already
>> enlarged to 4k entries and this is still small for 40k
>> kprobes.
>>
>> For example, when registering 40k probes on the hlist and
>> enabling 20k probes, perf tools shows still a lot of
>> cache-misses are on the get_kprobe.
>> ----
>> Samples: 633 of event 'cache-misses', Event count (approx.): 3414776
>> + 68.13% [k] get_kprobe
>> + 4.38% [k] ftrace_lookup_ip
>> + 2.54% [k] kprobe_ftrace_handler
>> ----
>>
>> Also, I found that the most of the kprobes are not hit.
>> In that case, to reduce cache-misses, we can reduce the
>> random memory access by introducing a per-cpu cache which
>> caches the address of frequently used kprobe data structure
>> and its probe address.
>>
>> With kpcache enabled, the get_kprobe_cached goes down to
>> around 4-5% of cache-misses with 20k probes.
>> ----
>> Samples: 729 of event 'cache-misses', Event count (approx.): 690125
>> + 14.49% [k] ftrace_lookup_ip
>> + 5.61% [k] kprobe_trace_func
>> + 5.17% [k] kprobe_ftrace_handler
>> + 4.62% [k] get_kprobe_cached
>> ----
>>
>> Of course this reduces the enabling time too.
>>
>> Without this fix (just enlarge hash table):
>> (2934 sec, 1 min intervals for each 2000 probes enabled)
>>
>> ----
>> Enabling trace events: start at 1393921862
>> 0 1393921864 a2mp_chan_alloc_skb_cb_38581
>> ...
>> 19999 1393924928 nfs4_open_confirm_done_11785
>> ----
>>
>> With this fix:
>> (2025 sec, 1 min intervals for each 2000 probes enabled)
>
> That's a nice speedup.

Thanks :)

>
> So I don't think this should be a Kconfig entry, just enable it
> unconditionally. That will further simplify the code.

Hmm, it consumes some amount of memory (36KB/core) just for the
case of several thousand of kprobes. On enterprise servers and desktop
it's OK, no problem. But I think, some embedded systems with small
resources will not want that. So, how about enabling Kconfig by default?

Thank you,

--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com