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    SubjectRe: [PATCH 3/3] workqueue: Schedule work on non-idle cpu instead of current one
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    On 25 September 2012 23:26, Tejun Heo <tj@kernel.org> wrote:
    > On Tue, Sep 25, 2012 at 04:06:08PM +0530, Viresh Kumar wrote:
    >> +config MIGRATE_WQ
    >> + bool "(EXPERIMENTAL) Migrate Workqueues to non-idle cpu"
    >> + depends on SMP && EXPERIMENTAL
    >> + help
    >> + Workqueues queues work on current cpu, if the caller haven't passed a
    >> + preferred cpu. This may wake up an idle CPU, which is actually not
    >> + required. This work can be processed by any CPU and so we must select
    >> + a non-idle CPU here. This patch adds in support in workqueue
    >> + framework to get preferred CPU details from the scheduler, instead of
    >> + using current CPU.
    >
    > I don't think it's a good idea to make behavior like this a config
    > option. The behavior difference is subtle and may induce incorrect
    > behavior.

    Ok. Will remove it.

    >> @@ -1066,8 +1076,9 @@ int queue_work(struct workqueue_struct *wq, struct work_struct *work)
    >> {
    >> int ret;
    >>
    >> - ret = queue_work_on(get_cpu(), wq, work);
    >> - put_cpu();
    >> + preempt_disable();
    >> + ret = queue_work_on(wq_select_cpu(), wq, work);
    >> + preempt_enable();
    >
    > First of all, I'm not entirely sure this is safe. queue_work() used
    > to *guarantee* that the work item would execute on the local CPU. I
    > don't think there are many which depend on that but I'd be surprised
    > if this doesn't lead to some subtle problems somewhere. It might not
    > be realistic to audit all users and we might have to just let it
    > happen and watch for the fallouts. Dunno, still wanna see some level
    > of auditing.

    Ok.

    > Also, I'm wondering why this is necessary at all for workqueues. For
    > schedule/queue_work(), you pretty much know the current cpu is not
    > idle. For delayed workqueue, sure but for immediate scheduling, why?

    This was done for below scenario:
    - A cpu has programmed a timer and is IDLE now.
    - CPU gets into interrupt handler due to timer and queues a work. As the
    CPU is currently IDLE, we should queue this work to some other CPU.

    I know this patch did migrate works in all cases. Will fix it by queuing work
    only for this case in V2.

    --
    viresh


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