linux-kernel - Re: [PATCH 3/3] lockdep: document MAX_LOCKDEP_CHAIN

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Message-ID: <45ceeb38-06c4-4b8a-8b3f-afe57c891f9a@redhat.com>
Date: Mon, 5 Aug 2024 21:27:52 -0400
From: Waiman Long <longman@...hat.com>
To: Carlos Llamas <cmllamas@...gle.com>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
 Huang Ying <ying.huang@...el.com>, "J. R. Okajima" <hooanon05g@...il.com>,
 Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 3/3] lockdep: document MAX_LOCKDEP_CHAIN_HLOCKS
 calculation

On 8/5/24 21:01, Carlos Llamas wrote:
> Add a comment to document the magic number '5' used in the calculation
> of MAX_LOCKDEP_CHAIN_HLOCKS. This number represents the estimated
> average depth (number of locks held) of a lock chain. This definition
> was added in commit 443cd507ce7f ("lockdep: add lock_class information
> to lock_chain and output it").
>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: J. R. Okajima <hooanon05g@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> ---
>   kernel/locking/lockdep_internals.h | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
> index bbe9000260d0..2b429ed103a8 100644
> --- a/kernel/locking/lockdep_internals.h
> +++ b/kernel/locking/lockdep_internals.h
> @@ -119,6 +119,7 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
>   
>   #define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)
>   
> +/* We estimate that a chain holds 5 locks on average. */
>   #define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5)
>   
>   extern struct lock_chain lock_chains[];

I think it is better to define another macro like

diff --git a/kernel/locking/lockdep_internals.h 
b/kernel/locking/lockdep_internals.h
index bbe9000260d0..8805122cc9f1 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -119,7 +119,8 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =

  #define MAX_LOCKDEP_CHAINS     (1UL << MAX_LOCKDEP_CHAINS_BITS)

-#define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5)
+#define AVG_LOCKDEP_CHAIN_DEPTH         5
+#define MAX_LOCKDEP_CHAIN_HLOCKS 
(MAX_LOCKDEP_CHAINS*AVG_LOCKDEP_CHAIN_DEPTH)

  extern struct lock_chain lock_chains[];

Cheers,
Longman