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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org, Zqiang <qiang1.zhang@intel.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH rcu 12/12] rcu: Add irqs-disabled indicator to expedited RCU CPU stall warnings
Date:   Mon, 20 Jun 2022 15:51:28 -0700
Message-Id: <20220620225128.3842050-12-paulmck@kernel.org>
In-Reply-To: <20220620224943.GA3841634@paulmck-ThinkPad-P17-Gen-1>
References: <20220620224943.GA3841634@paulmck-ThinkPad-P17-Gen-1>
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From: Zqiang <qiang1.zhang@intel.com>

If a CPU has interrupts disabled continuously starting before the
beginning of a given expedited RCU grace period, that CPU will not
execute that grace period's IPI handler.  This will in turn mean
that the ->cpu_no_qs.b.exp field in that CPU's rcu_data structure
will continue to contain the boolean value false.

Knowing whether or not a CPU has had interrupts disabled can be helpful
when debugging an expedited RCU CPU stall warning, so this commit
adds a "D" indicator expedited RCU CPU stall warnings that signifies
that the corresponding CPU has had interrupts disabled throughout.

This capability was tested as follows:

runqemu kvm slirp nographic qemuparams="-m 4096 -smp 4"  bootparams=
"isolcpus=2,3 nohz_full=2,3 rcu_nocbs=2,3 rcutree.dump_tree=1
rcutorture.stall_cpu_holdoff=30 rcutorture.stall_cpu=40
rcutorture.stall_cpu_irqsoff=1 rcutorture.stall_cpu_block=0
rcutorture.stall_no_softlockup=1" -d

The rcu_torture_stall() function ran on CPU 1, which displays the "D"
as expected given the rcutorture.stall_cpu_irqsoff=1 module parameter:

............
rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks:
{ 1-...D } 26467 jiffies s: 13317 root: 0x1/.
rcu: blocking rcu_node structures (internal RCU debug): l=1:0-1:0x2/.
Task dump for CPU 1:
task:rcu_torture_sta state:R  running task     stack:    0 pid:   76  ppid:     2 flags:0x00004008

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_exp.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 4c7037b507032..f092c7f18a5f3 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -637,10 +637,11 @@ static void synchronize_rcu_expedited_wait(void)
 					continue;
 				ndetected++;
 				rdp = per_cpu_ptr(&rcu_data, cpu);
-				pr_cont(" %d-%c%c%c", cpu,
+				pr_cont(" %d-%c%c%c%c", cpu,
 					"O."[!!cpu_online(cpu)],
 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
-					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
+					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
+					"D."[!!(rdp->cpu_no_qs.b.exp)]);
 			}
 		}
 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
-- 
2.31.1.189.g2e36527f23