Messages in this thread | | | Date | Fri, 3 May 2013 20:47:32 +0300 (EEST) | From | Julian Anastasov <> | Subject | Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper |
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Hello,
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) > { > #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU) > 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:
http://marc.info/?l=linux-netdev&m=136741839021257&w=2
I'll stick to this version, hope Peter Zijlstra agrees. Playing with PREEMPT_ACTIVE or another bit makes the things more complex.
To summarize:
- CONFIG_PREEMPT_RCU: - no empty functions called - CONFIG_DEBUG_ATOMIC_SLEEP can catch errors even for this case
- non-CONFIG_PREEMPT_RCU: - 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.
Regards
-- Julian Anastasov <ja@ssi.bg>
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