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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