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

To: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper
Cc: Julian Anastasov <ja@xxxxxx>, Simon Horman <horms@xxxxxxxxxxxx>, Eric Dumazet <eric.dumazet@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>, Dipankar Sarma <dipankar@xxxxxxxxxx>
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri, 3 May 2013 19:04:47 +0200
> 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)
> {
>       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 can do quite a number of superfluous rcu_read_unlock()/lock() pairs for
voluntary preemption kernels?
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