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

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

[Michal Hocko]

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@linux.vnet.ibm.com>
> AuthorDate: Wed, 30 Nov 2016 06:24:30 -0800
> Committer:  Paul E. McKenney <paulmck@linux.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

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

[Paul E. McKenney]

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@linux.vnet.ibm.com>
> > AuthorDate: Wed, 30 Nov 2016 06:24:30 -0800
> > Committer:  Paul E. McKenney <paulmck@linux.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

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

[Michal Hocko]

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@linux.vnet.ibm.com>
> > > AuthorDate: Wed, 30 Nov 2016 06:24:30 -0800
> > > Committer:  Paul E. McKenney <paulmck@linux.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