Re: [NET]: rt_check_expire() can take a long time, add a cond_resched()

On Fri, 16 Nov 2007 05:12:21 +0100
Eric Dumazet [off-list ref] wrote:

Eric Dumazet a écrit :

quoted

Arjan van de Ven a écrit :

quoted

On Thu, 15 Nov 2007 04:01:48 GMT
Linux Kernel Mailing List [off-list ref] wrote:

quoted

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
[off-list ref] AuthorDate: Wed Nov 14 16:14:05 2007 -0800
Committer:  David S. Miller [off-list ref]
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:

quoted

    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"

yeah the explicit reference makes the stack pointer tracking engine do a
commit I suspect which then also creates a data dependency in the code
flow.

however... this is likely a good argument for making cond_resched() as a
whole a #define (or inline) that does this test and then calls the out
of line code (which then doesn't need to retest, so it avoids the
double test)...




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