linux-kernel - Re: [RFC tip/locking/lockdep v5 15/17] lockdep: Reduce the size of lock

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Date:   Fri, 23 Feb 2018 12:38:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RFC tip/locking/lockdep v5 15/17] lockdep: Reduce the size of
 lock_list

On Thu, Feb 22, 2018 at 03:09:02PM +0800, Boqun Feng wrote:
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index a1f91f8680bd..3fce8dbf5091 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -186,11 +186,11 @@ struct lock_list {
>  	struct list_head		entry;
>  	struct lock_class		*class;
>  	struct stack_trace		trace;
> -	int				distance;
> +	u16				distance;
>  	/* bitmap of different dependencies from head to this */
> -	u16				dep;
> +	u8				dep;
>  	/* used by BFS to record whether this is picked as a recursive read */
> -	u16				is_rr;
> +	bool				is_rr;

Don't use bool, use u8 if you want a single byte. sizeof(_Bool) is part
of the architecture ABI and can be up to 4 bytes (well really anything,
but I've not seen it bigger than 4 bytes for any sane implementation).