From: Mike Galbraith <efault@gmx.de>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] sched: fix divide by zero at {thread_group,task}_times
Date: Wed, 08 Aug 2012 21:50:04 +0200 [thread overview]
Message-ID: <1344455404.2440.35.camel@marge.simpson.net> (raw)
In-Reply-To: <20120808092714.GA3580@redhat.com>
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
next prev parent reply other threads:[~2012-08-08 19:50 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-08 9:27 Stanislaw Gruszka
2012-08-08 19:50 ` Mike Galbraith [this message]
2012-08-08 20:08 ` Peter Zijlstra
2012-08-13 11:48 ` Stanislaw Gruszka
2012-08-13 13:59 ` Peter Zijlstra
2012-08-13 16:50 ` [tip:sched/urgent] sched: fix divide by zero at {thread_group, task}_times tip-bot for Stanislaw Gruszka
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