Re: [PATCH 6/7 v3] sched/fair: trigger the update of blocked load on newly idle cpu

[Valentin Schneider <valentin.schneider@arm.com>]

On 15/02/21 16:02, Vincent Guittot wrote:
> On Fri, 12 Feb 2021 at 20:19, Valentin Schneider
> <valentin.schneider@arm.com> wrote:
>> I don't think there is anything inherently wrong with it - the
>> nohz_idle_balance() call resulting from the kick_ilb() IPI will just bail
>> out due to the flags being cleared here. This wasn't immediately clear to
>> me however.
>
> In fact, I forgot to replace the WARN_ON in nohz_csd_func() by a
> simple return as reported by kernel test robot / oliver.sang@intel.com
>

Can't that actually be a problem? kick_ilb() says:

         * Access to rq::nohz_csd is serialized by NOHZ_KICK_MASK; he who sets
         * the first flag owns it; cleared by nohz_csd_func().

So if you have:

  kick_ilb() -> kicks CPU42

And then said CPU42 goes through, before nohz_csd_func(),:

  do_idle() -> nohz_run_idle_balance()

you could have yet another CPU do:

  kick_ilb() -> kicks CPU42

which would break rq->nohz_csd serialization.

>>
>> > +}
>> > +