linux-kernel - Re: [tip:core/rcu] sched,rcu: Make cond_resched() provide RCU quiescent state

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Message-Id: <20171024155214.GC3659@linux.vnet.ibm.com>
Date:   Tue, 24 Oct 2017 08:52:14 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rcu] sched,rcu: Make cond_resched() provide RCU
 quiescent state

On Tue, Oct 24, 2017 at 12:59:03PM +0200, Michal Hocko wrote:
> On Tue 24-10-17 03:02:09, tip-bot for Paul E. McKenney wrote:
> > Commit-ID:  f79c3ad6189624c3de0ad5521610c9e22a1c33cf
> > Gitweb:     https://git.kernel.org/tip/f79c3ad6189624c3de0ad5521610c9e22a1c33cf
> > Author:     Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > AuthorDate: Wed, 30 Nov 2016 06:24:30 -0800
> > Committer:  Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > CommitDate: Mon, 9 Oct 2017 14:25:17 -0700
> > 
> > sched,rcu: Make cond_resched() provide RCU quiescent state
> > 
> > There is some confusion as to which of cond_resched() or
> > cond_resched_rcu_qs() should be added to long in-kernel loops.
> > This commit therefore eliminates the decision by adding RCU quiescent
> > states to cond_resched().  This commit also simplifies the code that
> > used to interact with cond_resched_rcu_qs(), and that now interacts with
> > cond_resched(), to reduce its overhead.  This reduction is necessary to
> > allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
> > everywhere that cond_resched() is invoked.
> > 
> > Part of that reduction in overhead converts the jiffies_till_sched_qs
> > kernel parameter to read-only at runtime, thus eliminating the need for
> > bounds checking.
> 
> Thanks a lot Paul! I have just one question. Does the above mean that we
> can drop cond_resched_rcu_qs? Or there are still some scenarios when
> this is a better option?

Good point, and no time like the present!  Please see commits cc1c305fff49
("netfilter: Eliminate cond_resched_rcu_qs() in favor of cond_resched()")
through cca9c4def233 ("rcu: Eliminate the cond_resched_rcu_qs()
definition") in my -rcu tree:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

							Thanx, Paul