linux-kernel - Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.

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Date:   Tue, 9 Jan 2018 19:47:05 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Eryu Guan <eguan@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chris Metcalf <metcalf@...m.mit.edu>,
        David Laight <David.Laight@...LAB.COM>,
        Dmitry Vyukov <dvyukov@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.

Attached user space program I used to see the difference.
Usage:
	gcc -02 -o strscpy strscpy_test.c
	./strscpy {b|w} src_str_len count

src_str_len - length of source string in between 1-4096
count - how many strscpy() to execute.



Also I've noticed something strange. I'm not sure why, but certain
src_len values (e.g. 30) drives branch predictor crazy causing worse than usual results
for byte-at-a-time copy:

$ perf stat   ./strscpy b 29 10000000

 Performance counter stats for './strscpy b 29 10000000':

        165.354974      task-clock:u (msec)       #    0.999 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
                48      page-faults:u             #    0.290 K/sec                  
       640,475,981      cycles:u                  #    3.873 GHz                    
     2,500,090,080      instructions:u            #    3.90  insn per cycle         
       640,017,126      branches:u                # 3870.565 M/sec                  
             1,589      branch-misses:u           #    0.00% of all branches        

       0.165568346 seconds time elapsed



 Performance counter stats for './strscpy b 30 10000000':

        250.835659      task-clock:u (msec)       #    0.999 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
                46      page-faults:u             #    0.183 K/sec                  
       974,528,780      cycles:u                  #    3.885 GHz                    
     2,580,090,165      instructions:u            #    2.65  insn per cycle         
       660,017,211      branches:u                # 2631.273 M/sec                  
        14,488,234      branch-misses:u           #    2.20% of all branches        

       0.251147341 seconds time elapsed


 Performance counter stats for './strscpy b 31 10000000':

        176.598368      task-clock:u (msec)       #    0.997 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
                46      page-faults:u             #    0.260 K/sec                  
       681,367,948      cycles:u                  #    3.858 GHz                    
     2,660,090,092      instructions:u            #    3.90  insn per cycle         
       680,017,138      branches:u                # 3850.642 M/sec                  
             1,817      branch-misses:u           #    0.00% of all branches        

       0.177150181 seconds time elapsed


View attachment "strscpy_test.c" of type "text/x-csrc" (3292 bytes)