linux-kernel - Re: [PATCH] sched: fix divide by zero at {thread_group,task}

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Date:	Wed, 08 Aug 2012 21:50:04 +0200
From:	Mike Galbraith <efault@....de>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: fix divide by zero at {thread_group,task}_times

On Wed, 2012-08-08 at 11:27 +0200, Stanislaw Gruszka wrote: 
> On architectures where cputime_t is 64 bit type, is possible to trigger
> divide by zero on do_div(temp, (__force u32) total) line, if total is a
> non zero number but has lower 32 bit's zeroed. Removing casting is not
> a good solution since some do_div() implementations do cast to u32
> internally.

I wonder if do_div() vs div64_u64() makes a big difference.  I swiped
all the kernel bits, and took them to userland.  32bit built do_div()
and div64_u64() both sucked equally compared to 64bit.. on Q6600.  Too
bad P4 box kicked the bucket...

boring numbers..

marge:~ # gcc -O2 -m32 -DDO_DIV -g -o xx xx.c
marge:~ # taskset -c 0 perf stat -- taskset -c 3 ./xx

 Performance counter stats for 'taskset -c 3 ./xx':

      13067.570787 task-clock:HG             #    0.999 CPUs utilized          
              1117 context-switches:HG       #    0.085 K/sec                  
                11 CPU-migrations:HG         #    0.001 K/sec                  
               225 page-faults:HG            #    0.017 K/sec                  
       31211154646 cycles:HG                 #    2.388 GHz                     [50.00%]
   <not supported> stalled-cycles-frontend:HG
   <not supported> stalled-cycles-backend:HG
       69004479084 instructions:HG           #    2.21  insns per cycle         [75.00%]
       16004049251 branches:HG               # 1224.715 M/sec                   [75.00%]
            106179 branch-misses:HG          #    0.00% of all branches         [75.00%]

      13.074907234 seconds time elapsed

marge:~ # gcc -O2 -m32 -g -o xx xx.c
marge:~ # taskset -c 0 perf stat -- taskset -c 3 ./xx

 Performance counter stats for 'taskset -c 3 ./xx':

      13065.936608 task-clock:HG             #    0.999 CPUs utilized          
              1117 context-switches:HG       #    0.085 K/sec                  
                 3 CPU-migrations:HG         #    0.000 K/sec                  
               225 page-faults:HG            #    0.017 K/sec                  
       31202249014 cycles:HG                 #    2.388 GHz                     [50.01%]
   <not supported> stalled-cycles-frontend:HG
   <not supported> stalled-cycles-backend:HG
       69013127982 instructions:HG           #    2.21  insns per cycle         [75.00%]
       16003545136 branches:HG               # 1224.830 M/sec                   [75.00%]
            106156 branch-misses:HG          #    0.00% of all branches         [75.00%]

      13.073238280 seconds time elapsed

64bit is loads faster.

marge:~ # gcc -O2 -m64 -DDO_DIV -g -o xx xx.c
marge:~ # taskset -c 0 perf stat -- taskset -c 3 ./xx

 Performance counter stats for 'taskset -c 3 ./xx':

       5575.994225 task-clock:HG             #    0.999 CPUs utilized          
               473 context-switches:HG       #    0.085 K/sec                  
                 5 CPU-migrations:HG         #    0.001 K/sec                  
               251 page-faults:HG            #    0.045 K/sec                  
       13311760132 cycles:HG                 #    2.387 GHz                     [50.00%]
   <not supported> stalled-cycles-frontend:HG
   <not supported> stalled-cycles-backend:HG
        7003731700 instructions:HG           #    0.53  insns per cycle         [75.04%]
        1001242479 branches:HG               #  179.563 M/sec                   [75.05%]
             37489 branch-misses:HG          #    0.00% of all branches         [74.98%]

       5.579075361 seconds time elapsed

marge:~ # gcc -O2 -m64 -g -o xx xx.c
marge:~ # taskset -c 0 perf stat -- taskset -c 3 ./xx

 Performance counter stats for 'taskset -c 3 ./xx':

       5576.756281 task-clock:HG             #    0.999 CPUs utilized          
               482 context-switches:HG       #    0.086 K/sec                  
                 5 CPU-migrations:HG         #    0.001 K/sec                  
               251 page-faults:HG            #    0.045 K/sec                  
       13316232700 cycles:HG                 #    2.388 GHz                     [49.98%]
   <not supported> stalled-cycles-frontend:HG
   <not supported> stalled-cycles-backend:HG
        7006965347 instructions:HG           #    0.53  insns per cycle         [74.99%]
        1001095727 branches:HG               #  179.512 M/sec                   [75.04%]
             39450 branch-misses:HG          #    0.00% of all branches         [75.01%]

       5.580954618 seconds time elapsed

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