linux-kernel - Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain

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Date:   Tue, 4 Feb 2020 11:45:15 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain_hlocks entries

On 2/4/20 7:36 AM, Peter Zijlstra wrote:
> --- a/kernel/locking/lockdep_proc.c
> +++ b/kernel/locking/lockdep_proc.c
> @@ -278,9 +278,11 @@ static int lockdep_stats_show(struct seq
>  #ifdef CONFIG_PROVE_LOCKING
>  	seq_printf(m, " dependency chains:             %11lu [max: %lu]\n",
>  			lock_chain_count(), MAX_LOCKDEP_CHAINS);
> -	seq_printf(m, " dependency chain hlocks:       %11lu [max: %lu]\n",
> -			MAX_LOCKDEP_CHAIN_HLOCKS - nr_free_chain_hlocks,
> +	seq_printf(m, " dependency chain hlocks used:  %11lu [max: %lu]\n",
> +			MAX_LOCKDEP_CHAIN_HLOCKS - (nr_free_chain_hlocks - nr_lost_chain_hlocks),
>  			MAX_LOCKDEP_CHAIN_HLOCKS);
> +	seq_printf(m, " dependency chain hlocks free:  %11lu\n", nr_free_chain_hlocks);
> +	seq_printf(m, " dependency chain hlocks lost:  %11lu\n", nr_lost_chain_hlocks);

I do have some comments on this. There are three buckets now - free,
lost, used. They add up to MAX_LOCKDEP_CHAIN_HLOCKS. I don't think we
need to list all three. We can compute the third one by subtracting max
from the other two.

Something like:

diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index 14932ea50317..6fe6a21c58d3 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -278,9 +278,12 @@ static int lockdep_stats_show(struct seq_file *m,
void *v)
 #ifdef CONFIG_PROVE_LOCKING
        seq_printf(m, " dependency chains:             %11lu [max: %lu]\n",
                        lock_chain_count(), MAX_LOCKDEP_CHAINS);
-       seq_printf(m, " dependency chain hlocks:       %11lu [max: %lu]\n",
-                       MAX_LOCKDEP_CHAIN_HLOCKS - nr_free_chain_hlocks,
+       seq_printf(m, " dependency chain hlocks used:  %11lu [max: %lu]\n",
+                       MAX_LOCKDEP_CHAIN_HLOCKS -
+                       (nr_free_chain_hlocks + nr_lost_chain_hlocks),
                        MAX_LOCKDEP_CHAIN_HLOCKS);
+       seq_printf(m, " dependency chain hlocks lost:  %11lu\n",
+                       nr_lost_chain_hlocks);
 #endif
 
 #ifdef CONFIG_TRACE_IRQFLAGS

Cheers,
Longman