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    SubjectRe: [PATCH] sched/fair: Fix nohz.next_balance update
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    On 06/05/20 14:45, Vincent Guittot wrote:
    >> But then we may skip an update if we goto abort, no? Imagine we have just
    >> NOHZ_STATS_KICK, so we don't call any rebalance_domains(), and then as we
    >> go through the last NOHZ CPU in the loop we hit need_resched(). We would
    >> end in the abort part without any update to nohz.next_balance, despite
    >> having accumulated relevant data in the local next_balance variable.
    >
    > Yes but on the other end, the last CPU has not been able to run the
    > rebalance_domain so we must not move nohz.next_balance otherwise it
    > will have to wait for at least another full period
    > In fact, I think that we have a problem with current implementation
    > because if we abort because local cpu because busy we might end up
    > skipping idle load balance for a lot of idle CPUs
    >
    > As an example, imagine that we have 10 idle CPUs with the same
    > rq->next_balance which equal nohz.next_balance. _nohz_idle_balance
    > starts on CPU0, it processes idle lb for CPU1 but then has to abort
    > because of need_resched. If we update nohz.next_balance like
    > currently, the next idle load balance will happen after a full
    > balance interval whereas we still have 8 CPUs waiting for running an
    > idle load balance.
    >
    > My proposal also fixes this problem
    >

    That's a very good point; so with NOHZ_BALANCE_KICK we can reduce
    nohz.next_balance via rebalance_domains(), and otherwise we would only
    increase it if we go through a complete for_each_cpu() loop in
    _nohz_idle_balance().

    That said, if for some reason we keep bailing out of the loop, we won't
    push nohz.next_balance forward and thus may repeatedly nohz-balance only
    the first few CPUs in the NOHZ mask. I think that can happen if we have
    say 2 tasks pinned to a single rq, in that case nohz_balancer_kick() will
    kick a NOHZ balance whenever now >= nohz.next_balance.

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