netdev - Re: [PATCH v2 1/2] sched: Add cond_resched_rcu

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Date:	Fri, 3 May 2013 20:47:32 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Simon Horman <horms@...ge.net.au>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...hat.com>, 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>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper


	Hello,

On Fri, 3 May 2013, Paul E. McKenney wrote:

> OK, after getting some sleep, I might have located the root cause of
> my confusion yesterday.
> 
> The key point is that I don't understand why we cannot get the effect
> we are looking for with the following in sched.h (or wherever):
> 
> static inline int cond_resched_rcu(void)
> {
> #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
> 	rcu_read_unlock();
> 	cond_resched();
> 	rcu_read_lock();
> #endif
> }
> 
> This adds absolutely no overhead in non-debug builds of CONFIG_PREEMPT_RCU,
> does the checking in debug builds, and allows voluntary preemption in
> !CONFIG_PREEMPT_RCU builds.  CONFIG_PROVE_RCU builds will check for an
> (illegal) outer rcu_read_lock() in CONFIG_PREEMPT_RCU builds, and you
> will get "scheduling while atomic" in response to an outer rcu_read_lock()
> in !CONFIG_PREEMPT_RCU builds.
> 
> It also seems to me a lot simpler.
> 
> Does this work, or am I still missing something?

	It should work. It is a better version of
the 2nd variant I mentioned here:

http://marc.info/?l=linux-netdev&m=136741839021257&w=2

	I'll stick to this version, hope Peter Zijlstra
agrees. Playing with PREEMPT_ACTIVE or another bit makes
the things more complex.

	To summarize:

- CONFIG_PREEMPT_RCU:
	- no empty functions called
	- CONFIG_DEBUG_ATOMIC_SLEEP can catch errors even
	for this case

- non-CONFIG_PREEMPT_RCU:
	- rcu_read_lock and rcu_read_unlock are barrier(),
	so it expands just to cond_resched()

	I'll repeat the tests tomorrow and if there are
no problems will post official version after the merge window.

Regards

--
Julian Anastasov <ja@....bg>
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