Re: [PATCH 02/20 v3] sched/eevdf: Reset lag at wakeup when prev CPU has been idle

[Peter Zijlstra]

On Mon, Nov 09, 2026 at 04:27:14PM +0100, Vincent Guittot wrote:
> On Fri, 6 Nov 2026 at 17:22, Kayra Cizmeci <kayracizmeci@xxxxxxxxx> wrote:
> >
> > On Fri, Nov 06, 2026 at 04:19:29PM +0100, Vincent Guittot wrote:
> > > @@ -8376,6 +8383,9 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
> > >
> > >       dequeue_hierarchy(p, flags);
> > >
> > > +     if (!(flags & DEQUEUE_SAVE) && !cfs_rq->h_nr_queued)
> > > +             cfs_rq->idle_seq++;
> >
> > This needs WRITE_ONCE, no?  3/20 uses READ_ONCE().
> 
> I hesistated to make it WRITE_ONCE but i concluded that it was not
> strictly necessary because the READ_ONCE is only used to compare the
> value but not use it for any computation or sync so if the compiler
> wants to do any kind of optimization what we really care is : is it
> equal or not ? so any intermediate value that we would read would be
> ok

Right, so without WRITE_ONCE() the compiler is allowed to write the word
in random order byte writes. When comparing only cares about equality,
any intermediate messed up state is not equal. And since we were
incrementing, we're not supposed to be equal to any previous snapshot.

Therefore all is well.

That said; we should either document the above reasoning, or just do
WRITE_ONCE() and get all the sanitizers off our back, because I'm
certain that KCSAN is going to complain about this.