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: Thu, 2 May 2013 23:19:12 +0300 (EEST)

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():

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:

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


Julian Anastasov <ja@xxxxxx>
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