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

[Paul E. McKenney]

On Mon, Mar 18, 2024 at 10:35:44AM +0800, Z qiang wrote:
> > On Sun, Mar 17, 2024 at 10:31:22PM +0800, Z qiang wrote:
> > > > 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.  ;-)
> > > >
> > >
> > > Hi, Paul
> > >
> > > Would it be better to modify it as follows? set needwake not
> > > depend on lazy_jiffies,  if  ->lazy_jiffies be changed at runtime,
> > > and set it to zero, will miss the chance to wake up gp kthreads.
> > >
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index e7ac9138a4fd..aea2b71af36c 100644
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -299,11 +299,12 @@ 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)) {
> > >                 if (!rtpcp->urgent_gp)
> > >                         rtpcp->urgent_gp = 1;
> > >                 needwake = true;
> > > -               mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
> > > +               if (rtp->lazy_jiffies)
> > > +                       mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
> > >         }
> > >         raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
> > >         if (needwake)
> >
> > Interesting approach!
> >
> > But how does that avoid defeating laziness by doing the wakeup early?
> 
> Hello, Paul
> 
> Does this mean that if cblist is empty, we will miss the opportunity to
> enqueue the timer again?  If so, we can move mod_timer() outside
> the if condition.
> or I didn't understand your question?

Never mind!  I was getting confused, and forgetting that this code has
already seen a timeout.

Let's see what others think.

							Thanx, Paul

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