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    SubjectRe: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE
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    hi Peter, I guess you can receive the mail from me now,
    I have changed the mailbox to gmail.

    Oleg has already done much work for this patch, I am really obliged.
    please review this patch, thanks.

    on 09/10/2016 04:52 PM, Cheng Chao wrote:
    > For CONFIG_PREEMPT_NONE=y, when sched_exec() needs migration, sched_exec()
    > calls stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg).
    >
    > If the migration_cpu_stop() can not migrate,why do we call stop_one_cpu()?
    > It just makes the task TASK_UNINTERRUPTIBLE, wakes up the stopper thread,
    > executes migration_cpu_stop(), and the stopper thread wakes up the task.
    >
    > But in fact, all above works are almost useless(wasteful),the reason is
    > migration_cpu_stop() can not migrate. why? migration_cpu_stop() needs the
    > task is TASK_ON_RQ_QUEUED before it calls __migrate_task().
    >
    > This patch keeps the task TASK_RUNNING instead of TASK_UNINTERRUPTIBLE,
    > so the migration_cpu_stop() can do useful works.
    >
    > Signed-off-by: Cheng Chao <cs.os.kernel@gmail.com>
    > ---
    > kernel/stop_machine.c | 11 +++++++++++
    > 1 file changed, 11 insertions(+)
    >
    > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
    > index 4a1ca5f..41aea5e 100644
    > --- a/kernel/stop_machine.c
    > +++ b/kernel/stop_machine.c
    > @@ -126,6 +126,17 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
    > cpu_stop_init_done(&done, 1);
    > if (!cpu_stop_queue_work(cpu, &work))
    > return -ENOENT;
    > +
    > +#if defined(CONFIG_PREEMPT_NONE)
    > + /*
    > + * Makes the stopper thread run as soon as possible.
    > + * And if the caller is TASK_RUNNING, keeps the caller TASK_RUNNING.
    > + * It's special useful for some callers which are expected to be
    > + * TASK_ON_RQ_QUEUED.
    > + * sched_exec does benefit from this improvement.
    > + */
    > + schedule();
    > +#endif
    > wait_for_completion(&done.completion);
    > return done.ret;
    > }
    >

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