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    SubjectRe: [NET]: rt_check_expire() can take a long time, add a cond_resched()
    On Fri, 16 Nov 2007 05:12:21 +0100
    Eric Dumazet <dada1@cosmosbay.com> wrote:

    > Eric Dumazet a écrit :
    > > Arjan van de Ven a écrit :
    > >> On Thu, 15 Nov 2007 04:01:48 GMT
    > >> Linux Kernel Mailing List <linux-kernel@vger.kernel.org> 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
    > >>> <dada1@cosmosbay.com> AuthorDate: Wed Nov 14 16:14:05 2007 -0800
    > >>> Committer: David S. Miller <davem@davemloft.net>
    > >>> 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"

    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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