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    SubjectRe: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist
    On 01/17, Srivatsa Vaddagiri wrote:
    > On Tue, Jan 16, 2007 at 04:27:25PM +0300, Oleg Nesterov wrote:
    > > > I meant issuing kthread_stop() in DOWN_PREPARE so that worker
    > > > thread exits itself (much before CPU is actually brought down).
    > >
    > > Deadlock if work_struct re-queues itself.
    > Are you referring to the problem you described here?
    > If so, then it can easily be prevented by having run_workqueue() check for
    > kthread_should_stop() in its while loop?

    flush_workqueue() also calls run_workqueue().

    > > > workqueue_cpu_callback()
    > > > {
    > > >
    > > > CPU_DEAD:
    > > > /* threads are still frozen at this point */
    > > > take_over_work();
    > >
    > > No, we can't move a currently executing work. This will break flush_workxxx().
    > What do you mean by "currently" executing work? worker thread executing
    > some work on the cpu? That is not possible, because all threads are
    > frozen at this point. There cant be any ongoing flush_workxxx() as well
    > because of this, which should avoid breaking flush_workxxx() ..

    work->func() sleeps/freezed. We can't move the rest of pending jobs before
    it completes. This will break flush_workxxx. And no, this is not because
    we use barriers now.

    > 1st method, what you are suggesting:
    > - Needs separate bitmap(s), cpu_populated_map and possible another
    > for create_workqueue()?
    > - flush_workqueue() traverses thr a separate bitmap
    > cpu_populated_map (separate from the online map) while
    > create_workqueue() traverses the other bitmap

    Yes, we need the additional bitmap. This is optimization, we can just use
    cpu_possible_map. create_workqueue() can use cpu_online_map + "int new_cpu".

    Yes, this is a complication. But still this is much simpler (IMHO) than
    we have now. And imho better.

    > 2nd method:
    > - Avoids the need for maintenance of separate bitmaps (uses
    > existing cpu_online_map). All functions can safely use
    > the online_map w/o any races. Personally this is why I like
    > this approach.
    > - Needs changes in worker_thread to exit right after it comes
    > out of refrigerator.
    > I havent made any changes as per 2nd method to see the resulting code
    > size, so I cant comment on code sizes.

    Yes, yes, yes, let's see the code first! :) Then we can compare.
    Right now:
    - cpu-hotplug doesn't use freezer yet
    - all ideas about using it to improve workqueue.c were wrong

    > Another point is that once we create code as in 1st method, which
    > maintains separate bitmaps, that will easily get replicated (over time)
    > to other subsystems. Is that a good thing?

    Honestly, I can't understand your point. Why it will get replicated?
    Because another subsystem will need cpu_populated_map too? We can remove
    "static" and move cpu_populated_map to kernel/cpu.c then.

    Btw, I agree it is good to have a sleeping lock to protect cpu_online_map.
    But it should be separate from workqueue_mutex, and it is not needed for
    create/destroy/flush funcs.


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      Last update: 2007-01-17 17:13    [W:0.023 / U:3.352 seconds]
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