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    SubjectRe: [PATCH] mm, vmstat: Use cond_resched only when !CONFIG_PREEMPT
    On Tue, 31 May 2011 09:58:40 +0600
    Rakib Mullick <rakib.mullick@gmail.com> wrote:

    > On Tue, May 31, 2011 at 9:18 AM, KAMEZAWA Hiroyuki
    > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > On Tue, 31 May 2011 09:13:47 +0600
    > > Rakib Mullick <rakib.mullick@gmail.com> wrote:
    > >
    > >> On Tue, May 31, 2011 at 5:38 AM, KAMEZAWA Hiroyuki
    > >> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > >> > On Mon, 30 May 2011 22:59:04 +0600
    > >> > Rakib Mullick <rakib.mullick@gmail.com> wrote:
    > >> >
    > >> >> commit 468fd62ed9 (vmstats: add cond_resched() to refresh_cpu_vm_stats()) added cond_resched() in refresh_cpu_vm_stats. Purpose of that patch was to allow other threads to run in non-preemptive case. This patch, makes sure that cond_resched() gets called when !CONFIG_PREEMPT is set. In a preemptiable kernel we don't need to call cond_resched().
    > >> >>
    > >> >> Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
    > >> >
    > >> > Hmm, what benefit do we get by adding this extra #ifdef in the code directly ?
    > >> > Other cond_resched() callers are not guilty in !CONFIG_PREEMPT ?
    > >> >
    > >> Well, in preemptible kernel this context will get preempted if
    > >> requires, so we don't need cond_resched(). If you checkout the git log
    > >> of the mentioned commit, you'll find the explanation. It says:
    > >>         "Adding a cond_resched() to allow other threads to run in the
    > >> non-preemptive
    > >>     case."
    > >>
    > >
    > > IOW, my question is "why only this cond_resched() should be fixed ?"
    >
    > cond_resched() forces this thread to be scheduled. I'm just trying
    > pointing out the use of cond_resched(), until unless I'm not missing
    > anything.
    >
    > > What's bad with all cond_resched() in the kernel as no-op in CONFIG_PREEMPT ?
    > >
    > cond_resched() basically checks whether it needs to be scheduled or
    > not. But, we know in advance that we don't need cond_resched in
    > CONFIG_PREEMPT.
    >
    > Thanks,

    Then just remove _all_ cond_resched() by defining noop function in header.
    Please don't fix in ugly way.

    #ifdef CONFIG_PEERMPT
    static void cond_resched()
    {
    }
    #endif

    Thanks,
    -Kame




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