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: Peter Zijlstra <peterz@xxxxxxxxxxxxx>, 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: Julian Anastasov <ja@xxxxxx>
Date: Fri, 3 May 2013 20:47:32 +0300 (EEST)

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)
> {
>       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:

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

        To summarize:

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

        - 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.


Julian Anastasov <ja@xxxxxx>
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