linux-kernel - Re: [tip:sched/core] sched/core: Avoid _cond

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Message-Id: <20160923144708.GC14933@linux.vnet.ibm.com>
Date:   Fri, 23 Sep 2016 07:47:08 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mikulas Patocka <mpatocka@...hat.com>, hpa@...or.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        torvalds@...ux-foundation.org, mingo@...nel.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org, oleg@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y

On Fri, Sep 23, 2016 at 10:44:42AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 22, 2016 at 01:28:17PM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 22, 2016 at 08:42:28PM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> > > > I've found a document that says that cond_resched() is needed on
> > > > preemptible kernels to mark RCU quiescent states:
> > > > 
> > > > https://lwn.net/Articles/603252/
> > > > 
> > > > Paul, is cond_resched() still needed on current RCU implementation? Or not?
> > > 
> > > No, we ripped that out. See 4a81e8328d37 ("rcu: Reduce overhead of
> > > cond_resched() checks for RCU")
> > 
> > I would instead say that we changed the algorithm to reduce the overhead
> > in the common case, but yes, this commit did change things quite a bit.
> 
> Sure, but critical point (for this discussion) is that cond_resched() no
> longer implies anything RCU, it really only is a voluntary reschedule
> point.

Agreed, cond_resched_rcu_qs() is what you use to do both a cond_resched()
and an RCU quiescent state.

							Thanx, Paul