linux-kernel - Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

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Date:   Fri, 3 Mar 2017 09:17:37 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com, kernel-team@....com,
        Michal Hocko <mhocko@...nel.org>,
        Nikolay Borisov <nborisov@...e.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

On Thu, Mar 02, 2017 at 02:40:31PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 01, 2017 at 01:28:43PM +0100, Peter Zijlstra wrote:
> > On Wed, Mar 01, 2017 at 02:43:23PM +0900, Byungchul Park wrote:
> 
> > > It's an optimization, but very essential and important optimization.
> 
> Since its not for correctness, please put it in a separate patch with a
> good Changelog, the below would make a good beginning of that.

OK. I will do it.

> Also, I feel, the source comments can be improved.
> 
> > >           in hlocks[]
> > >           ------------
> > >           A gen_id (4) --+
> > >                          | previous gen_id
> > >           B gen_id (3) <-+
> > >           C gen_id (3)
> > >           D gen_id (2)
> > > oldest -> E gen_id (1)
> > > 
> > >           in xhlocks[]
> > >           ------------
> > >        ^  A gen_id (4) prev_gen_id (3: B's gen id)
> > >        |  B gen_id (3) prev_gen_id (3: C's gen id)
> > >        |  C gen_id (3) prev_gen_id (2: D's gen id)
> > >        |  D gen_id (2) prev_gen_id (1: E's gen id)
> > >        |  E gen_id (1) prev_gen_id (NA)
> > > 
> > > Let's consider the case that the gen id of xlock to commit is 3.
> > > 
> > > In this case, it's engough to generate 'the xlock -> C'. 'the xlock -> B'
> > > and 'the xlock -> A' are unnecessary since it's covered by 'C -> B' and
> > > 'B -> A' which are already generated by original lockdep.
> > > 
> > > I use the prev_gen_id to avoid adding this kind of redundant
> > > dependencies. In other words, xhlock->prev_gen_id >= xlock->hlock.gen_id
> > > means that the previous lock in hlocks[] is able to handle the
> > > dependency on its commit stage.

> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index a95e5d1..7baea89 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1860,6 +1860,17 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
>  		}
>  	}
>  
> +	/*
> +	 * Is the <prev> -> <next> redundant?
> +	 */
> +	this.class = hlock_class(prev);
> +	this.parent = NULL;
> +	ret = check_noncircular(&this, hlock_class(next), &target_entry);
> +	if (!ret) /* exists, redundant */
> +		return 2;
> +	if (ret < 0)
> +		return print_bfs_bug(ret);
> +
>  	if (!*stack_saved) {
>  		if (!save_trace(&trace))
>  			return 0;

This whoud be very nice if you allow to add this code. However, prev_gen_id
thingy is still useful, the code above can achieve it though. Agree?