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    SubjectRe: [PATCH 2/7] sched: Fix picking a task switching on other cpu (__ARCH_WANT_UNLOCKED_CTXSW)
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    В Сб, 20/09/2014 в 20:54 +0200, Peter Zijlstra пишет:
    > On Sat, Sep 20, 2014 at 08:33:26PM +0200, Peter Zijlstra wrote:
    > > On Sat, Sep 20, 2014 at 08:51:22PM +0400, Kirill Tkhai wrote:
    > > > From: Kirill Tkhai <ktkhai@parallels.com>
    > > >
    > > > We may pick a task which is in context_switch() on other cpu at the moment.
    > > > Parallel using of a single stack by two processes is not a good idea.
    > >
    > > Please elaborate on who exactly that might happen. Its best to have
    > > comprehensive changelogs for issues that fix races.
    >
    > FWIW IIRC we can remove UNLOCKED_CTXSW from IA64 and I forgot if I
    > audited MIPS, but I suspect we can (and should) remove it there too.
    >
    > That would make this exception go away and clean up some of this ugly.

    Yeah, you've said me about IA64:

    http://www.spinics.net/lists/linux-ia64/msg10229.html

    It's about 10 years since the logic, which was documented in ia64
    header, has been removed. It looks like, ia64 maintainers are not
    interested much...

    ***

    To do not to start a new message. I've found the above when I was
    analysing if the optimisation below is OK (assume, we have accessor
    cpu_relax__while_on_cpu()):

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 7d0d023..8d765ba 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -1699,8 +1699,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    goto stat;

    #ifdef CONFIG_SMP
    - cpu_relax__while_on_cpu(p);
    -
    p->sched_contributes_to_load = !!task_contributes_to_load(p);
    p->state = TASK_WAKING;

    @@ -1708,6 +1706,9 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    p->sched_class->task_waking(p);

    cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
    +
    + cpu_relax__while_on_cpu(p);
    +
    if (task_cpu(p) != cpu) {
    wake_flags |= WF_MIGRATED;
    set_task_cpu(p, cpu);
    Looks like, now problem here. Task p is dequeued, we can set sched_contributes_to_load and state
    here, also task_waking does not produce problems, only arithmetics is there. select_task_rq()
    is R/O function.

    Now I'm testing this.

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