linux-kernel - Re: [PATCH rcu 3/9] rcu/tree: Reduce wake up for synchronize

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Message-ID: <CAFwiDX-x4=_N2szD2QZhAEkU+kx_eTscz2bcyHqikHq7m=rkpQ@mail.gmail.com>
Date: Fri, 7 Jun 2024 07:21:55 +0530
From: Neeraj upadhyay <neeraj.iitr10@...il.com>
To: paulmck@...nel.org
Cc: Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com, rostedt@...dmis.org, 
	"Joel Fernandes (Google)" <joel@...lfernandes.org>, Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH rcu 3/9] rcu/tree: Reduce wake up for synchronize_rcu()
 common case

On Thu, Jun 6, 2024 at 11:42 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Thu, Jun 06, 2024 at 11:28:07AM +0530, Neeraj upadhyay wrote:
> > On Wed, Jun 5, 2024 at 10:05 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> > >
> > > Le Tue, Jun 04, 2024 at 03:23:49PM -0700, Paul E. McKenney a écrit :
> > > > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > > >
> > > > In the synchronize_rcu() common case, we will have less than
> > > > SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
> > > > is pointless just to free the last injected wait head since at that point,
> > > > all the users have already been awakened.
> > > >
> > > > Introduce a new counter to track this and prevent the wakeup in the
> > > > common case.
> > > >
> > > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > > Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > ---
> > > >  kernel/rcu/tree.c | 35 ++++++++++++++++++++++++++++++-----
> > > >  kernel/rcu/tree.h |  1 +
> > > >  2 files changed, 31 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 6ba36d9c09bde..2fe08e6186b4d 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -96,6 +96,7 @@ static struct rcu_state rcu_state = {
> > > >       .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
> > > >       .srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
> > > >               rcu_sr_normal_gp_cleanup_work),
> > > > +     .srs_cleanups_pending = ATOMIC_INIT(0),
> > > >  };
> > > >
> > > >  /* Dump rcu_node combining tree at boot to verify correct setup. */
> > > > @@ -1633,8 +1634,11 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > > >        * the done tail list manipulations are protected here.
> > > >        */
> > > >       done = smp_load_acquire(&rcu_state.srs_done_tail);
> > > > -     if (!done)
> > > > +     if (!done) {
> > > > +             /* See comments below. */
> > > > +             atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
> > >
> > > This condition is not supposed to happen. If the work is scheduled,
> > > there has to be a wait_queue in rcu_state.srs_done_tail. And decrementing
> > > may make things worse.
> > >
> >
> > I also don't see a scenario where this can happen. However, if we are
> > returning from here, given that for every queued work we do an
> > increment of rcu_state.srs_cleanups_pending, I think it's safer to
> > decrement in this
> > case, as that counter tracks only the work queuing and execution counts.
> >
> >     atomic_inc(&rcu_state.srs_cleanups_pending);
> >     if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work))
> >         atomic_dec(&rcu_state.srs_cleanups_pending);
>
> Linus Torvald's general rule is that if you cannot imagine how a bug
> can happen, don't attempt to clean up after it.  His rationale (which
> is *almost* always a good one) is that not knowing how the bug happens
> means that attempts to clean up will usually just make matters worse.
> And all too often, the clean-up code makes debugging more difficult.
>

Ok. Thanks for sharing this info!

> One example exception to this rule is when debug-objects detects a
> duplicate call_rcu().  In that case, we ignore that second call_rcu().
> But the reason is that experience has shown that the usual cause really
> is someone doing a duplicate call_rcu(), and also that ignoring the
> second call_rcu() makes debugging easier.
>
> So what is it that Frederic and I are missing here?
>

Maybe nothing. As kworker context does not modify srs_done_tail and
invalid values
of srs_done_tail  can only be caused by the GP kthread manipulations
of  srs_done_tail , my thought here was, we can keep the pending
rcu_sr_normal_gp_cleanup_work count consistent with the number of
queue_work() and kworker executions, even when we see unexpected
srs_done_tail values like these. However, as you described the general rule
is to not attempt any clean up for such scenarios.

Thanks
Neeraj



>                                                         Thanx, Paul
>
> > Thanks
> > Neeraj
> >
> > > So this should be:
> > >
> > > if (WARN_ON_ONCE(!done))
> > >    return;
> > >
> > > Thanks.
> > >
> >