netdev - Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections

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Date:	Fri, 26 Apr 2013 08:15:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Simon Horman <horms@...ge.net.au>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Julian Anastasov <ja@....bg>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when
 dumping connections


* Simon Horman <horms@...ge.net.au> wrote:

> This avoids the situation where a dump of a large number of connections
> may prevent scheduling for a long time while also avoiding excessive
> calls to rcu_read_unlock() and rcu_read_lock().
> 
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Julian Anastasov <ja@....bg>
> Signed-off-by: Simon Horman <horms@...ge.net.au>
> ---
>  net/netfilter/ipvs/ip_vs_conn.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_conn.c b/net/netfilter/ipvs/ip_vs_conn.c
> index a083bda..42a7b33 100644
> --- a/net/netfilter/ipvs/ip_vs_conn.c
> +++ b/net/netfilter/ipvs/ip_vs_conn.c
> @@ -975,8 +975,7 @@ static void *ip_vs_conn_array(struct seq_file *seq, loff_t pos)
>  				return cp;
>  			}
>  		}
> -		rcu_read_unlock();
> -		rcu_read_lock();
> +		cond_resched_rcu_lock();
>  	}
>  
>  	return NULL;
> @@ -1015,8 +1014,7 @@ static void *ip_vs_conn_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>  			iter->l = &ip_vs_conn_tab[idx];
>  			return cp;
>  		}
> -		rcu_read_unlock();
> -		rcu_read_lock();
> +		cond_resched_rcu_lock();

Feel free to route this via the networking tree.

Note that this change isn't a pure clean-up but has functional effects as 
well: on !PREEMPT or PREEMPT_VOLUNTARY kernels it will add in a potential 
cond_resched() - while previously it would only rcu unlock and relock 
(which in itself isn't scheduling).

This should probably be pointed out in the changelog.

Thanks,

	Ingo
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