Eric Dumazet a écrit :
Arjan van de Ven a écrit :
On Thu, 15 Nov 2007 04:01:48 GMT
Linux Kernel Mailing List <[email protected]> wrote:
Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d90bf5a976793edfa88d3bb2393f0231eb8ce1e5
Commit: d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 Parent:
66ba886254edbbd9442d30f1eef6f6fb0145027d Author: Eric Dumazet
<[email protected]> AuthorDate: Wed Nov 14 16:14:05 2007 -0800
Committer: David S. Miller <[email protected]>
CommitDate: Wed Nov 14 16:14:05 2007 -0800
[NET]: rt_check_expire() can take a long time, add a
cond_resched() On commit 39c90ece7565f5c47110c2fa77409d7a9478bd5b:
When the IP route cache is big, rt_check_expire() can take a long
time to run. (default settings : 20% of the hash table is scanned at
each invocation)
Adding cond_resched() helps giving cpu to higher priority
tasks if
necessary.
Using a "if (need_resched())" test before calling
"cond_resched();" is necessary to avoid spending too much time doing
the resched check.
int __sched cond_resched(void)
{
if (need_resched() && .....
somehow I wonder why the second if() is useful at all; it's another
spot for a branch predictor to miss... and a void function call is
really really cheap...
Its not that cheap. The ChangeLog included my own numbers, on a Pentium
M machine. (i686, 1.6 GHz, 1.5 GB ram)
Without "if (need_resched())" (so calling need_resched() X.XXX.XXX
times), each run takes 88ms
With the extra check (and *much* less function calls), each run takes 25ms
Looking at cond_resched(), I think the extra cost comes from
"mov %esp,%edx ; and $0xffffe000,%edx" (current_thread_info())
I dont have oprofile numbers yet, but I suspect CPU may have some delays
to compute this pointer value, since %esp is probably 'busy' because
of the preceding "call"
(In the case the "if (need_resched())" is done in rt_check_expire(),
compiler moves this pointer computation (current_thread_info()) out of the loop)
c055f926 <cond_resched>:
c055f926: 89 e2 mov %esp,%edx
c055f928: 81 e2 00 e0 ff ff and $0xffffe000,%edx
c055f92e: 8b 42 08 mov 0x8(%edx),%eax
c055f931: a8 04 test $0x4,%al
c055f933: 74 1a je c055f94f <cond_resched+0x29>
c055f935: f6 42 17 10 testb $0x10,0x17(%edx)
c055f939: 75 14 jne c055f94f <cond_resched+0x29>
c055f93b: 83 3d 00 80 7c c0 01 cmpl $0x1,0xc07c8000
c055f942: 75 0b jne c055f94f <cond_resched+0x29>
c055f944: e8 2b 80 bb ff call c0117974 <__cond_resched>
c055f949: b8 01 00 00 00 mov $0x1,%eax
c055f94e: c3 ret
c055f94f: 31 c0 xor %eax,%eax
c055f951: c3 ret
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