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Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper

To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper
Cc: Simon Horman <horms@xxxxxxxxxxxx>, Eric Dumazet <eric.dumazet@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>, Dipankar Sarma <dipankar@xxxxxxxxxx>
From: Julian Anastasov <ja@xxxxxx>
Date: Thu, 2 May 2013 13:06:30 +0300 (EEST)
        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@xxxxxx>
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