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

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Date:   Tue,  4 Jul 2023 16:26:15 +0800
From:   Zqiang <qiang.zhang1211@...il.com>
To:     paulmck@...nel.org, frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, qiang.zhang1211@...il.com
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] srcu: Make srcu_might_be_idle() take early return if rcu_gp_is_normal() return true

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;
 	/* 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