Date: Mon, 13 Apr 2015 19:02:38 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: rusty@rustcorp.com.au, mathieu.desnoyers@efficios.com, oleg@redhat.com,
        paulmck@linux.vnet.ibm.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, andi@firstfloor.org, rostedt@goodmis.org,
        tglx@linutronix.de, laijs@cn.fujitsu.com, linux@horizon.com
Subject: Re: [PATCH v5 00/10] latched RB-trees and __module_address()
Message-ID: <20150413170237.GA26512@gmail.com>
References: <20150413141126.756350256@infradead.org>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> This series is aimed at making __module_address() go fast(er).
> 
> The reason for doing so is that most stack unwinders use kernel_text_address()
> to validate each frame. Perf and ftrace (can) end up doing a lot of stack
> traces from performance sensitive code.
> 
> On the way there it:
>  - annotates and sanitizes module locking
>  - introduces the latched RB-tree
>  - employs it to make __module_address() go fast.
> 
> I've build and boot tested this on x86_64 with modules and lockdep
> enabled.  Performance numbers (below) are done with lockdep disabled.
> 
> As previously mentioned; the reason for writing the latched RB-tree as generic
> code is mostly for clarity/documentation purposes; as there are a number of
> separate and non trivial bits to the complete solution.
> 
> As measued on my ivb-ep system with 84 modules loaded; prior to 
> patching the test module (below) reports (cache hot, performance 
> cpufreq):
> 
>           avg +- stdev
> Before:   611 +- 10 [ns] per __module_address() call
> After:     17 +-  5 [ns] per __module_address() call
> 
> PMI measurements for a cpu running loops in a module (also [ns]):
> 
> Before: Mean: 2719 +- 1, Stdev: 214, Samples: 40036
> After:  Mean:  947 +- 0, Stdev: 132, Samples: 40037

Those are some pretty impressive speedups!

I suspect the 900 nsecs residual PMI overhead is due to other, overly 
bloated PMI (perf) processing costs?

> Note; I have also tested things like: perf record -a -g modprobe 
> mod_test, to make 'sure' to hit some of the more interesting paths.
> 
> Changes since last time:
> 
>  - reworked generic latch_tree API (Lai Jiangshan)
>  - reworked module bounds (me)
>  - reworked all the testing code (not included)
> 
> Rusty, please consider merging this (for 4.2, I know its the merge 
> window, no rush)

So modulo the mostly trivial feedback I gave, it looks all good to me 
as well, feel free to also add my:

  Reviewed-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo
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