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

To: Julian Anastasov <ja@xxxxxx>
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: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Fri, 3 May 2013 09:30:45 -0700
On Fri, May 03, 2013 at 10:52:36AM +0300, Julian Anastasov wrote:
>       Hello,
> On Thu, 2 May 2013, Paul E. McKenney wrote:
> > mainline, and missed the one that you added.  Revisiting that, a
> > question:
> > 
> > > +#ifdef CONFIG_PREEMPT_RCU
> > > +#define PREEMPT_RCU_OFFSET     1
> > 
> > Does this really want to be "1" instead of PREEMPT_OFFSET?
>       In this case when CONFIG_PREEMPT_RCU is enabled
> we (RCU) do not touch the preempt counters. Instead, the units
> are accounted in current->rcu_read_lock_nesting:
> #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
> __rcu_read_lock:
>       current->rcu_read_lock_nesting++;
>       and the path is __might_sleep -> preempt_count_equals ->
> rcu_preempt_depth
>       For now both places do not use PREEMPT_OFFSET:
> - #define inc_preempt_count() add_preempt_count(1)
> - __rcu_read_lock: current->rcu_read_lock_nesting++;
>       so, ... it does not matter much for me. In short,
> the trick is in preempt_count_equals() where preempt_offset
> is a combination of preempt count and RCU preempt depth:
> #define PREEMPT_RCU_OFFSET      (0 /* preempt */ + 1 /* RCU */)
> #else
> #endif
>       Let me know for your preference about this definition...

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)

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()

It also seems to me a lot simpler.

Does this work, or am I still missing something?

                                                        Thanx, Paul

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