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

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Message-ID: <20171024163419.ijd2u5taa6k5qs4r@dhcp22.suse.cz>
Date:   Tue, 24 Oct 2017 18:34:19 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 24-10-17 08:52:14, Paul E. McKenney wrote:
> 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:

cool stuff. Thanks!

-- 
Michal Hocko
SUSE Labs