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In-Reply-To: <986428d9-5d82-463c-908b-b88435d53e79@paulmck-laptop>
From: Z qiang <qiang.zhang1211@gmail.com>
Date: Sun, 17 Mar 2024 22:31:22 +0800
Message-ID: <CALm+0cWf9ODsxodfm+3JHciuEAe4=0fA3R4n4rhjh7XZzHonNQ@mail.gmail.com>
Subject: Re: [PATCH] rcu-tasks: Remove unnecessary lazy_jiffies in call_rcu_tasks_generic_timer()
To: paulmck@kernel.org
Cc: frederic@kernel.org, neeraj.upadhyay@kernel.org, joel@joelfernandes.org, 
	rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

>
> 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@gmail.com>
> > > > ---
> > > >  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)

Thanks
Zqiang


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