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

[Z qiang]

>
> 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.

Thanks
Zqiang

>
>                                                         Thanx, Paul
>
> >       /* If the local srcu_data structure has callbacks, not idle.  */
> >       sdp = raw_cpu_ptr(ssp->sda);
> >       spin_lock_irqsave_rcu_node(sdp, flags);
> > --
> > 2.17.1
> >