linux-kernel - Re: [PATCH] rcu-scale: rcu-scale returns one less than the real number of gps in the dmesg report.

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Message-ID: <20210622174633.GD4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 22 Jun 2021 10:46:33 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Jiangong.Han" <jiangong.han@...driver.com>
Cc:     dave@...olabs.net, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, linux-kernel@...r.kernel.org,
        rcu@...r.kernel.org
Subject: Re: [PATCH] rcu-scale: rcu-scale returns one less than the real
 number of gps in the dmesg report.

On Tue, Jun 22, 2021 at 06:37:08PM +0800, Jiangong.Han wrote:
> The dmesg report on rcu-scale shows there are N grace periods, and gps
> are listed from 0 to N.
> This commit make writer_n_durations stores the counts of gps,
> and shows there are N+1 gps, change the listed gps index begin from 0 to N.
> 
> From
>     [ 8306.087880] rcu-scale: writer 0 gps: 133
>     ......
>     [ 8307.864630] rcu-scale:    0 writer-duration:     0 44003961
>     [ 8307.935711] rcu-scale:    0 writer-duration:     1 32003582
>     ......
>     [ 8316.472860] rcu-scale:    0 writer-duration:   132 28004391
>     [ 8316.538498] rcu-scale:    0 writer-duration:   133 27996410
> 
> to
>     [ 8306.087880] rcu-scale: writer 0 gps: 134
>     ......
>     [ 8307.864630] rcu-scale:    0 writer-duration:     0 44003961
>     [ 8307.935711] rcu-scale:    0 writer-duration:     1 32003582
>     ......
>     [ 8316.472860] rcu-scale:    0 writer-duration:   132 28004391
>     [ 8316.538498] rcu-scale:    0 writer-duration:   133 27996410
> 
> Signed-off-by: Jiangong.Han <jiangong.han@...driver.com>

Much better, thank you!

As usual, I could not resist the urge to wordsmith a bit, so could
you please check the version below to make sure that I didn't mess
something up?

							Thanx, Paul


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

commit 4c154a9103443b22aef8667ab64278d12372e64f
Author: Jiangong.Han <jiangong.han@...driver.com>
Date:   Tue Jun 22 18:37:08 2021 +0800

    rcuscale: Console output claims too few grace periods
    
    The rcuscale console output claims N grace periods, numbered from zero
    to N, which means that there were really N+1 grace periods.  The root
    cause of this bug is that rcu_scale_writer() stores the number of the
    last grace period (numbered from zero) into writer_n_durations[me]
    instead of the number of grace periods.  This commit therefore assigns
    the actual number of grace periods to writer_n_durations[me], and also
    makes the corresponding adjustment to the loop outputting per-grace-period
    measurements.
    
    Sample of old console output:
        rcu-scale: writer 0 gps: 133
        ......
        rcu-scale:    0 writer-duration:     0 44003961
        rcu-scale:    0 writer-duration:     1 32003582
        ......
        rcu-scale:    0 writer-duration:   132 28004391
        rcu-scale:    0 writer-duration:   133 27996410
    
    Sample of new console output:
        rcu-scale: writer 0 gps: 134
        ......
        rcu-scale:    0 writer-duration:     0 44003961
        rcu-scale:    0 writer-duration:     1 32003582
        ......
        rcu-scale:    0 writer-duration:   132 28004391
        rcu-scale:    0 writer-duration:   133 27996410
    
    Signed-off-by: Jiangong.Han <jiangong.han@...driver.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index dca51fe9c73f..2cc34a22a506 100644
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -487,7 +487,7 @@ rcu_scale_writer(void *arg)
 	if (gp_async) {
 		cur_ops->gp_barrier();
 	}
-	writer_n_durations[me] = i_max;
+	writer_n_durations[me] = i_max + 1;
 	torture_kthread_stopping("rcu_scale_writer");
 	return 0;
 }
@@ -561,7 +561,7 @@ rcu_scale_cleanup(void)
 			wdpp = writer_durations[i];
 			if (!wdpp)
 				continue;
-			for (j = 0; j <= writer_n_durations[i]; j++) {
+			for (j = 0; j < writer_n_durations[i]; j++) {
 				wdp = &wdpp[j];
 				pr_alert("%s%s %4d writer-duration: %5d %llu\n",
 					scale_type, SCALE_FLAG,