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    SubjectRe: [PATCH] sched/fair: Fix nohz.next_balance update
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    On 04/05/20 16:17, Vincent Guittot wrote:
    > On Sun, 3 May 2020 at 10:34, Peng Liu <iwtbavbm@gmail.com> wrote:
    >>
    >> commit c5afb6a87f23 ("sched/fair: Fix nohz.next_balance update")
    >> During idle load balance, this_cpu(ilb) do load balance for the other
    >> idle CPUs, also gather the earliest (nohz.)next_balance.
    >>
    >> Since commit:
    >> 'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
    >>
    >> We update nohz.next_balance like this:
    >>
    >> _nohz_idle_balance() {
    >> for_each_cpu(nohz.idle_cpus_mask) {
    >> rebalance_domains() {
    >> update nohz.next_balance <-- compare and update
    >> }
    >> }
    >> rebalance_domains(this_cpu) {
    >> update nohz.next_balance <-- compare and update
    >> }
    >> update nohz.next_balance <-- unconditionally update
    >> }
    >>
    >> For instance, nohz.idle_cpus_mask spans {cpu2,3,5,8}, and this_cpu is
    >> cpu5. After the above loop we could gather the earliest *next_balance*
    >> among {cpu2,3,8}, then rebalance_domains(this_cpu) update
    >> nohz.next_balance with this_rq->next_balance, but finally overwrite
    >> nohz.next_balance with the earliest *next_balance* among {cpu2,3,8},
    >> we may end up with not getting the earliest next_balance.
    >>
    >> Since we can gather all the updated rq->next_balance, including this_cpu,
    >> in _nohz_idle_balance(), it's safe to remove the extra lines in
    >> rebalance_domains() which are originally intended for this_cpu. And
    >> finally the updating only happen in _nohz_idle_balance().
    >
    > I'm not sure that's always true. Nothing prevents nohz_idle_balance()
    > to return false . Then run_rebalance_domains() calls
    > rebalance_domains(this_rq ,SCHED_IDLE) outside _nohz_idle_balance().
    > In this case we must keep the code in rebalance_domains().
    >
    > For example when the tick is not stopped when entering idle. Or when
    > need_resched() returns true.
    >

    I had missed that, good points.

    > So instead of removing the code from rebalance_domains, you should
    > move the one in _nohz_idle_balance() to make sure that the "if
    > (likely(update_next_balance)) ..." is called before calling
    > rebalance_domains for the local cpu
    >

    Why not just get rid of the update in _nohz_idle_balance() entirely then?
    The nohz.next_balance update in rebalance_domains() will always happen if
    it is required (and we have idle == CPU_IDLE), so the extra update in
    _nohz_idle_balance() doesn't seem to be any useful.

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