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

Ingo Molnar
2014-04-24T09:01:45Z

* 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.

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

Thanks,

Ingo