Re: [PATCH] srcu: Make srcu_might_be_idle() take early return if rcu_gp_is_normal() return true

[Joel Fernandes]

On Fri, Jul 7, 2023 at 1:16 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Jul 7, 2023 at 12:05 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Fri, Jul 07, 2023 at 06:28:29PM +0800, Z qiang wrote:
> > > >
> > > > On Tue, Jul 04, 2023 at 04:26:15PM +0800, Zqiang wrote:
> > > > > When invoke synchronize_srcu(), in the srcu_might_be_idle(), the current
> > > > > CPU's sdp->lock will be acquired to check whether there are pending
> > > > > callbacks in the sdp->srcu_cblist, if there are no pending callbacks,
> > > > > probabilistically probe global state to decide whether to convert to
> > > > > synchronize_srcu_expedited() call. however, for the rcupdate.rcu_normal=1
> > > > > kernels and after the rcu_set_runtime_mode() is called, invoke the
> > > > > rcu_gp_is_normal() is always return true, this mean that invoke the
> > > > > synchronize_srcu_expedited() always fall back to synchronize_srcu(),
> > > > > so there is no need to acquire sdp->lock to check sdp->srcu_cblist and
> > > > > probe global state in srcu_might_be_idle().
> > > > >
> > > > > This commit therefore make srcu_might_be_idle() return immediately if the
> > > > > rcu_gp_is_normal() return true.
> > > > >
> > > > > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > > > > ---
> > > > >  kernel/rcu/srcutree.c | 2 ++
> > > > >  1 file changed, 2 insertions(+)
> > > > >
> > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > index 20d7a238d675..aea49cb60a45 100644
> > > > > --- a/kernel/rcu/srcutree.c
> > > > > +++ b/kernel/rcu/srcutree.c
> > > > > @@ -1172,6 +1172,8 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
> > > > >       unsigned long tlast;
> > > > >
> > > > >       check_init_srcu_struct(ssp);
> > > > > +     if (rcu_gp_is_normal())
> > > > > +             return false;
> > > >
> > > > Again, thank you for looking into SRCU!
> > > >
> > > > I am not at all enthusiastic about this one.  With this change, the name
> > > > srcu_might_be_idle() is no longer accurate.  Yes, the name could change,
> > > > but any name would be longer and more confusing.
> > > >
> > > > So unless there is a measureable benefit to this one on a production
> > > > workload, I cannot justify taking it.
> > > >
> > > > Is there a measureable benefit?
> > >
> > > Hi, Paul
> > >
> > > I only find that for Preempt-RT kernel,  the rcu_normal_after_boot is
> > > set by default:
> > > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
> > > This affects only rcu but also srcu, this make the synchronize_srcu() and
> > > synchronize_srcu_expedited() always fall back to __synchronize_srcu(ssp, true),
> > > this means that call the srcu_might_be_idle() is meaningless.
> >
> > I do understand that the current setup favors default kernel builds at
> > runtime by a few low-cost instructions, and that your change favors,
> > as you say, kernels built for real-time, kernels built for certain types
> > of HPC workloads, and all kernels during a small time during boot.
> >
> > My question is instead whether any of this makes a measureable difference
> > at the system level.
> >
> > My guess is "no, not even close", but the way to convince me otherwise
> > would be to actually run the workload and kernels on real hardware and
> > provide measurements showing a statistically significant difference that
> > the workload(s) in question care(s) about.
> >
> > So what can you show me?
> >
> > And srcu_might_be_idle() is not meaningless in that situation, just
> > ignored completely.  And that is in fact the nature and purpose of the
> > C-language || operator.  ;-)
>
> I agree with Paul, without any evidence of improvement, optimizing an
> obvious slow path is a NAK.

Just to clarify, when I meant improvement I meant any kind (ex. better
for maintenance, better performance numbers etc.). In this case, the
extra 2 lines does not seem to buy much AFAICS.

Thanks.