Re: [External] Re: Fwd: WARNING: CPU: 13 PID: 3837105 at kernel/sched/sched.h:1561 __cfsb_csd_unthrottle+0x149/0x160

[Hao Jia]

On 2023/9/8 Tim Chen wrote:
> On Thu, 2023-09-07 at 16:59 +0800, Hao Jia wrote:
> > On 2023/9/5 Peter Zijlstra wrote:
> > > On Thu, Aug 31, 2023 at 04:48:29PM +0800, Hao Jia wrote:
> > >
> > > > If I understand correctly, rq->clock_update_flags may be set to
> > > > RQCF_ACT_SKIP after __schedule() holds the rq lock, and sometimes the rq
> > > > lock may be released briefly in __schedule(), such as newidle_balance(). At
> > > > this time Other CPUs hold this rq lock, and then calling
> > > > rq_clock_start_loop_update() may trigger this warning.
> > > >
> > > > This warning check might be wrong. We need to add assert_clock_updated() to
> > > > check that the rq clock has been updated before calling
> > > > rq_clock_start_loop_update().
> > > >
> > > > Maybe some things can be like this?
> > >
> > > Urgh, aside from it being white space mangled, I think this is entirely
> > > going in the wrong direction.
> > >
> > > Leaking ACT_SKIP is dodgy as heck.. it's entirely too late to think
> > > clearly though, I'll have to try again tomorrow.
>
> I am trying to understand why this is an ACT_SKIP leak.
> Before call to __cfsb_csd_unthrottle(), is it possible someone
> else lock the runqueue, set ACT_SKIP and release rq_lock?
> And then that someone never update the rq_clock?

Yes, we want to set rq->clock_update_flags to RQCF_ACT_SKIP to avoid 
updating the rq clock multiple times in __cfsb_csd_unthrottle().

But now we find ACT_SKIP leak, so we cannot unconditionally set 
rq->clock_update_flags to RQCF_ACT_SKIP in rq_clock_start_loop_update().


> > Hi Peter,
> >
> > Do you think this fix method is correct? Or should we go back to the
> > beginning and move update_rq_clock() from unthrottle_cfs_rq()?
> If anyone who locked the runqueue set ACT_SKIP also will update rq_clock,
> I think your change is okay.  Otherwise rq_clock could be missing update.
>
> Thanks.
>
> Tim