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    SubjectRe: [PATCH] srcu: Make srcu_might_be_idle() take early return if rcu_gp_is_normal() return true
    >
    > 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@gmail.com>
    > > ---
    > > 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
    > >

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