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

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Message-ID: <20140424090134.GC7768@gmail.com>
Date:	Thu, 24 Apr 2014 11:01:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>, x86@...nel.org,
	Steven Rostedt <rostedt@...dmis.org>, fche@...hat.com,
	mingo@...hat.com, systemtap@...rceware.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH -tip v9 25/26] kprobes: Introduce kprobe cache to reduce
 cache misshits


* Masami Hiramatsu <masami.hiramatsu.pt@...achi.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
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