linux-kernel - Re: [PATCH ipvs-next v3 1/2] sched: add cond_resched

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Date:	Thu, 23 May 2013 13:30:19 +0200
From:	Pablo Neira Ayuso <pablo@...filter.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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH ipvs-next v3 1/2] sched: add cond_resched_rcu() helper

On Wed, May 22, 2013 at 02:50:31PM +0900, Simon Horman wrote:
> This is intended for use in loops which read data protected by RCU and may
> have a large number of iterations.  Such an example is dumping the list of
> connections known to IPVS: ip_vs_conn_array() and ip_vs_conn_seq_next().
> 
> The benefits are for CONFIG_PREEMPT_RCU=y where we save CPU cycles
> by moving rcu_read_lock and rcu_read_unlock out of large loops
> but still allowing the current task to be preempted after every
> loop iteration for the CONFIG_PREEMPT_RCU=n case.
> 
> The call to cond_resched() is not needed when CONFIG_PREEMPT_RCU=y.
> Thanks to Paul E. McKenney for explaining this and for the
> final version that checks the context with CONFIG_DEBUG_ATOMIC_SLEEP=y
> for all possible configurations.
> 
> The function can be empty in the CONFIG_PREEMPT_RCU case,
> rcu_read_lock and rcu_read_unlock are not needed in this case
> because the task can be preempted on indication from scheduler.
> Thanks to Peter Zijlstra for catching this and for his help
> in trying a solution that changes __might_sleep.
> 
> Initial cond_resched_rcu_lock() function suggested by Eric Dumazet.

Applied, thanks.
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