linux-kernel - Re: [PATCH tip/core/rcu 06/22] rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu

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Message-Id: <20180423131227.GM26088@linux.vnet.ibm.com>
Date:   Mon, 23 Apr 2018 06:12:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel.opensrc@...il.com
Subject: Re: [PATCH tip/core/rcu 06/22] rcu: Rename cond_resched_rcu_qs() to
 cond_resched_tasks_rcu_qs()

On Mon, Apr 23, 2018 at 02:48:55PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 23, 2018 at 05:32:23AM -0700, Paul E. McKenney wrote:
> > On Mon, Apr 23, 2018 at 10:53:47AM +0200, Peter Zijlstra wrote:
> > > On Sun, Apr 22, 2018 at 07:32:11PM -0700, Paul E. McKenney wrote:
> > > > Commit e31d28b6ab8f ("trace: Eliminate cond_resched_rcu_qs() in favor
> > > > of cond_resched()") substituted cond_resched() for the earlier call
> > > > to cond_resched_rcu_qs().  However, the new-age cond_resched() does
> > > > not do anything to help RCU-tasks grace periods because (1) RCU-tasks
> > > > is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
> > > > complete no-op when preemption is enabled.  This situation results
> > > > in hangs when running the trace benchmarks.
> > > 
> > > But a few patches earlier you tried to address that..
> > 
> > Yes.  And all this patch does is change the name to better match
> > what this primitive actually does.
> > 
> > I could merge the two patches, if that would help.
> 
> I understood the earlier patch was actually changing cond_resched() to
> also work for task-rcu; which resulted in it acually not being a no-op
> for PREEMPT anymore.
> 
> But then here you say it doesn't because it's a no-op.
> 
> It's not about the rename.. It is about one patch changing PREEMPT
> cond_resched to not be a no-op and you here justifying things because
> PREEMPT cond_resched it a no-op.

So would it help if I instead said in the commit log that it is a
no-op in production builds of the Linux kernel?

							Thanx, Paul