linux-kernel - Re: [PATCH v2 1/2] sched: Add cond_resched_rcu

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Date:	Thu, 2 May 2013 23:19:12 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Simon Horman <horms@...ge.net.au>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...hat.com>, lvs-devel@...r.kernel.org,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper


	Hello,

On Thu, 2 May 2013, Paul E. McKenney wrote:

> > 	Only the new cond_resched_rcu() macro provides
> > PREEMPT_ACTIVE flag to skip the rcu_preempt_sleep_check()
> > call. The old macros provide locked=0 as you noticed. Does it
> > answer your question or I'm missing something?
> 
> PREEMPT_ACTIVE's value is usually 0x10000000.  Did it change
> since 3.9?  If not, rcu_sleep_check(preempt_offset & PREEMPT_ACTIVE)
> is the same as rcu_sleep_check(0).

	Yes, the different platforms use different bit,
that is why I mentioned about my failed attempt at
changing hardirq.h. PREEMPT_ACTIVE is always != 0.

	But I don't understand what do you mean by
'preempt_offset & PREEMPT_ACTIVE' being always 0.
It is always 0 for cond_resched(), cond_resched_lock()
and cond_resched_softirq(), not for cond_resched_rcu():

(PREEMPT_ACTIVE | PREEMPT_RCU_OFFSET) & PREEMPT_ACTIVE
should give PREEMPT_ACTIVE, not 0. We have 2 cases in
rcu_sleep_check() for the if:

1. !(PREEMPT_ACTIVE) => FALSE for cond_resched_rcu
2. !(0) => TRUE for other cond_resched_* macros

	On x86 the code is:

__might_sleep:
        pushl   %ebp    #
        testl   $268435456, %ecx        #, preempt_offset
...
        jne     .L240   #,
	// rcu_lock_map checked when PREEMPT_ACTIVE is missing
.L240:
	// rcu_bh_lock_map checked

Regards

--
Julian Anastasov <ja@....bg>
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