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

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Message-ID: <20171024105903.iavajcc3cnljyyud@dhcp22.suse.cz>
Date:   Tue, 24 Oct 2017 12:59:03 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        paulmck@...ux.vnet.ibm.com
Cc:     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 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?
-- 
Michal Hocko
SUSE Labs