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

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Date:	Thu, 2 May 2013 13:06:30 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Simon Horman <horms@...ge.net.au>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"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,
	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 Thu, 2 May 2013, Peter Zijlstra wrote:

> On Wed, May 01, 2013 at 09:22:08PM +0300, Julian Anastasov wrote:
> > > +extern int __cond_resched_rcu(void);
> > > +
> > > +#define cond_resched_rcu() ({			\
> > > +	__might_sleep(__FILE__, __LINE__, 0);	\
> > 
> > 	I see your goal. But digging into __might_sleep()
> > I see that rcu_sleep_check() will scream for the non-preempt
> > case because we are under rcu_read_lock.
> 
> 
> #ifdef CONFIG_PREEMPT_RCU
> #define PREEMPT_RCU_OFFSET 0
> #else
> #define PREEMPT_RCU_OFFSET 1
> #endif
> 
> #define cond_resched_rcu() ({	\
> 	__might_sleep(__FILE__, __LINE__, PREEMPT_RCU_OFFSET);	\
> 	__cond_resched_rcu();	\
> })
> 
> Should work I think..

	Looks like CONFIG_DEBUG_ATOMIC_SLEEP selects
CONFIG_PREEMPT_COUNT, so PREEMPT_RCU_OFFSET should be
1 in all cases because preempt_disable() adds 1, while
for CONFIG_PREEMPT_RCU case rcu_preempt_depth() should
return 1:

#ifdef CONFIG_PREEMPT_RCU
#define PREEMPT_RCU_OFFSET 1
#else
#define PREEMPT_RCU_OFFSET PREEMPT_CHECK_OFFSET
#endif

	Now the remaining part is to fix rcu_sleep_check() for
the non-preempt case. As there are no nesting depths in this
case, I don't see a solution so far. We can provide
some argument to rcu_preempt_sleep_check to compare
depth with preempt_count() but currently I don't know
how to differentiate cond_resched_lock() from cond_resched_rcu()
when calling __might_sleep, in both cases we provide
PREEMPT_OFFSET. May be some trick is needed here without
adding new arg to __might_sleep, so that we can properly
check for rcu_lock_map.

Regards

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