Re: [PATCH] rcu-tasks: Remove unnecessary lazy_jiffies in call_rcu_tasks_generic_timer()

[Paul E. McKenney]

On Wed, Mar 13, 2024 at 12:49:50PM +0800, Z qiang wrote:
> >
> > On Tue, Mar 12, 2024 at 07:35:24PM +0800, Zqiang wrote:
> > > The rcu_tasks_percpu structure's->lazy_timer is queued only when
> > > the rcu_tasks structure's->lazy_jiffies is not equal to zero in
> > > call_rcu_tasks_generic(), if the lazy_timer callback is invoked,
> > > that means the lazy_jiffes is not equal to zero, this commit
> > > therefore remove lazy_jiffies check in call_rcu_tasks_generic_timer().
> > >
> > > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > > ---
> > >  kernel/rcu/tasks.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index b1254cf3c210..439e0b9a2656 100644
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -299,7 +299,7 @@ static void call_rcu_tasks_generic_timer(struct timer_list *tlp)
> > >
> > >       rtp = rtpcp->rtpp;
> > >       raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
> > > -     if (!rcu_segcblist_empty(&rtpcp->cblist) && rtp->lazy_jiffies) {
> > > +     if (!rcu_segcblist_empty(&rtpcp->cblist)) {
> >
> > Good eyes!
> >
> > But did you test with something like a WARN_ON_ONCE(rtp->lazy_jiffies)?
> 
> Hi, Paul
> 
> +  if (!rcu_segcblist_empty(&rtpcp->cblist) &&
> !WARN_ON_ONCE(!rtp->lazy_jiffies))
> 
> I've done tests like this:
> 
> 1. runqemu nographic kvm slirp qemuparams="-smp 4 -m 2048M -drive
> file=$PWD/share.img,if=virtio"
> bootparams="rcupdate.rcu_tasks_trace_lazy_ms=0" -d
> 
> 2.  insmod torture.ko
>      insmod rcutorture.ko torture_type=tasks-tracing fwd_progress=4
> 
> 3. bpftrace -e 't:timer:timer_expire_entry /args->function ==
> kaddr("call_rcu_tasks_generic_timer")/
>                                             {
> printf("comm:%s,cpu:%d,stack:%s,func:%s\n", comm, cpu, kstack,
> ksym(args->function)); }'
> 
> The call_rcu_tasks_generic_timer() has never been executed.

Very good!

Then if we get a couple of acks or reviews from the others acknowledging
that if they ever make ->lazy_jiffies be changeable at runtime, they
will remember to do something to adjust this logic appropriately, I will
take it.  ;-)

							Thanx, Paul

> Thanks
> Zqiang
> 
> 
> >
> >                                                 Thanx, Paul
> >
> > >               if (!rtpcp->urgent_gp)
> > >                       rtpcp->urgent_gp = 1;
> > >               needwake = true;
> > > --
> > > 2.17.1
> > >