linux-kernel - Re: [PATCH] rcu/nocb: Fix dump_tree hierarchy print always active

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Date:   Sun, 6 Oct 2019 18:26:25 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Stefan Reiter <stefan@...aker.at>
Cc:     rcu@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix dump_tree hierarchy print always active

On Fri, Oct 04, 2019 at 03:24:02PM -0700, Paul E. McKenney wrote:
> On Fri, Oct 04, 2019 at 07:49:10PM +0000, Stefan Reiter wrote:
> > Commit 18cd8c93e69e ("rcu/nocb: Print gp/cb kthread hierarchy if
> > dump_tree") added print statements to rcu_organize_nocb_kthreads for
> > debugging, but incorrectly guarded them, causing the function to always
> > spew out its message.
> > 
> > This patch fixes it by guarding both pr_alert statements with dump_tree,
> > while also changing the second pr_alert to a pr_cont, to print the
> > hierarchy in a single line (assuming that's how it was supposed to
> > work).
> > 
> > Fixes: 18cd8c93e69e ("rcu/nocb: Print gp/cb kthread hierarchy if dump_tree")
> > Signed-off-by: Stefan Reiter <stefan@...aker.at>
> 
> Queued for testing and review, thank you!

And here is an updated version to make the special case of a nocb GP
kthread having no other nocb CB kthreads look less strange.  Does this
work for you?

							Thanx, Paul

------------------------------------------------------------------------

commit e6223b0705369750990c32ddc80251942e61be30
Author: Stefan Reiter <stefan@...aker.at>
Date:   Fri Oct 4 19:49:10 2019 +0000

    rcu/nocb: Fix dump_tree hierarchy print always active
    
    Commit 18cd8c93e69e ("rcu/nocb: Print gp/cb kthread hierarchy if
    dump_tree") added print statements to rcu_organize_nocb_kthreads for
    debugging, but incorrectly guarded them, causing the function to always
    spew out its message.
    
    This patch fixes it by guarding both pr_alert statements with dump_tree,
    while also changing the second pr_alert to a pr_cont, to print the
    hierarchy in a single line (assuming that's how it was supposed to
    work).
    
    Fixes: 18cd8c93e69e ("rcu/nocb: Print gp/cb kthread hierarchy if dump_tree")
    Signed-off-by: Stefan Reiter <stefan@...aker.at>
    [ paulmck: Make single-nocbs-CPU GP kthreads look less erroneous. ]
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d5334e4..d43f4e0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2295,6 +2295,8 @@ static void __init rcu_organize_nocb_kthreads(void)
 {
 	int cpu;
 	bool firsttime = true;
+	bool gotnocbs = false;
+	bool gotnocbscbs = true;
 	int ls = rcu_nocb_gp_stride;
 	int nl = 0;  /* Next GP kthread. */
 	struct rcu_data *rdp;
@@ -2317,21 +2319,31 @@ static void __init rcu_organize_nocb_kthreads(void)
 		rdp = per_cpu_ptr(&rcu_data, cpu);
 		if (rdp->cpu >= nl) {
 			/* New GP kthread, set up for CBs & next GP. */
+			gotnocbs = true;
 			nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
 			rdp->nocb_gp_rdp = rdp;
 			rdp_gp = rdp;
-			if (!firsttime && dump_tree)
-				pr_cont("\n");
-			firsttime = false;
-			pr_alert("%s: No-CB GP kthread CPU %d:", __func__, cpu);
+			if (dump_tree) {
+				if (!firsttime)
+					pr_cont("%s\n", gotnocbscbs
+							? "" : " (self only)");
+				gotnocbscbs = false;
+				firsttime = false;
+				pr_alert("%s: No-CB GP kthread CPU %d:",
+					 __func__, cpu);
+			}
 		} else {
 			/* Another CB kthread, link to previous GP kthread. */
+			gotnocbscbs = true;
 			rdp->nocb_gp_rdp = rdp_gp;
 			rdp_prev->nocb_next_cb_rdp = rdp;
-			pr_alert(" %d", cpu);
+			if (dump_tree)
+				pr_cont(" %d", cpu);
 		}
 		rdp_prev = rdp;
 	}
+	if (gotnocbs && dump_tree)
+		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
 }
 
 /*