On 15/02/21 16:02, Vincent Guittot wrote:
> On Fri, 12 Feb 2021 at 20:19, Valentin Schneider
> <[email protected]> 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 / [email protected]
>
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.
>>
>> > +}
>> > +
