netdev - Re: [PATCH v1 net-next 2/6] af_unix: Save the number of loops in inflight graph.

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Date: Wed, 8 May 2024 10:05:25 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <pabeni@...hat.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<kuni1840@...il.com>, <kuniyu@...zon.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v1 net-next 2/6] af_unix: Save the number of loops in inflight graph.

From: Paolo Abeni <pabeni@...hat.com>
Date: Wed, 08 May 2024 12:08:48 +0200
> On Tue, 2024-05-07 at 09:11 -0700, Kuniyuki Iwashima wrote:
> > From: Paolo Abeni <pabeni@...hat.com>
> > Date: Tue, 07 May 2024 15:54:33 +0200
> > > On Fri, 2024-05-03 at 15:31 -0700, Kuniyuki Iwashima wrote:
> > > > unix_walk_scc_fast() calls unix_scc_cyclic() for every SCC so that we
> > > > can make unix_graph_maybe_cyclic false when all SCC are cleaned up.
> > > > 
> > > > If we count the number of loops in the graph during Tarjan's algorithm,
> > > > we need not call unix_scc_cyclic() in unix_walk_scc_fast().
> > > > 
> > > > Instead, we can just decrement the number when calling unix_collect_skb()
> > > > and update unix_graph_maybe_cyclic based on the count.
> > > > 
> > > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > > ---
> > > >  net/unix/garbage.c | 19 +++++++++++--------
> > > >  1 file changed, 11 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/net/unix/garbage.c b/net/unix/garbage.c
> > > > index 1f8b8cdfcdc8..7ffb80dd422c 100644
> > > > --- a/net/unix/garbage.c
> > > > +++ b/net/unix/garbage.c
> > > > @@ -405,6 +405,7 @@ static bool unix_scc_cyclic(struct list_head *scc)
> > > >  
> > > >  static LIST_HEAD(unix_visited_vertices);
> > > >  static unsigned long unix_vertex_grouped_index = UNIX_VERTEX_INDEX_MARK2;
> > > > +static unsigned long unix_graph_circles;
> > > >  
> > > >  static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_index,
> > > >  			    struct sk_buff_head *hitlist)
> > > > @@ -494,8 +495,8 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde
> > > >  
> > > >  		if (scc_dead)
> > > >  			unix_collect_skb(&scc, hitlist);
> > > > -		else if (!unix_graph_maybe_cyclic)
> > > > -			unix_graph_maybe_cyclic = unix_scc_cyclic(&scc);
> > > > +		else if (unix_scc_cyclic(&scc))
> > > > +			unix_graph_circles++;
> > > >  
> > > >  		list_del(&scc);
> > > >  	}
> > > > @@ -509,7 +510,7 @@ static void unix_walk_scc(struct sk_buff_head *hitlist)
> > > >  {
> > > >  	unsigned long last_index = UNIX_VERTEX_INDEX_START;
> > > >  
> > > > -	unix_graph_maybe_cyclic = false;
> > > > +	unix_graph_circles = 0;
> > > >  
> > > >  	/* Visit every vertex exactly once.
> > > >  	 * __unix_walk_scc() moves visited vertices to unix_visited_vertices.
> > > > @@ -524,13 +525,12 @@ static void unix_walk_scc(struct sk_buff_head *hitlist)
> > > >  	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
> > > >  	swap(unix_vertex_unvisited_index, unix_vertex_grouped_index);
> > > >  
> > > > +	unix_graph_maybe_cyclic = !!unix_graph_circles;
> > > >  	unix_graph_grouped = true;
> > > >  }
> > > >  
> > > >  static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
> > > >  {
> > > > -	unix_graph_maybe_cyclic = false;
> > > > -
> > > >  	while (!list_empty(&unix_unvisited_vertices)) {
> > > >  		struct unix_vertex *vertex;
> > > >  		struct list_head scc;
> > > > @@ -546,15 +546,18 @@ static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
> > > >  				scc_dead = unix_vertex_dead(vertex);
> > > >  		}
> > > >  
> > > > -		if (scc_dead)
> > > > +		if (scc_dead) {
> > > >  			unix_collect_skb(&scc, hitlist);
> > > > -		else if (!unix_graph_maybe_cyclic)
> > > > -			unix_graph_maybe_cyclic = unix_scc_cyclic(&scc);
> > > > +			unix_graph_circles--;
> > > 
> > > Possibly WARN_ON_ONCE(unix_graph_circles < 0) ?
> > 
> > Will add in v2.
> > 
> > > 
> > > I find this patch a little scaring - meaning I can't understand it
> > > fully,
> > > I'm wondering if it would make any sense to postpone this patch
> > > to the next cycle?
> > 
> > It's fine by me to postpone patch 2 - 5, but it would be appreciated
> > if patch 1 makes it to this cycle.
> 
> Yes, patch 1 looks fine and safe to me. Feel free to re-submit that one
> individually right now, with my Acked-by tag.

Thanks, will do!