linux-kernel - Re: [PATCH] srcu: Make srcu_might_be_idle() take early return if rcu_gp_is

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Message-ID: <77d16cbb-b28c-4b13-aecb-9ad787559557@paulmck-laptop>
Date:   Thu, 6 Jul 2023 10:08:27 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang.zhang1211@...il.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...il.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?

							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
>