linux-kernel - Re: [PATCH tip/core/rcu 2/3] rcu: Provide polling interfaces for Tiny RCU grace periods

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Date:   Mon, 22 Mar 2021 20:00:35 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 2/3] rcu: Provide polling interfaces for
 Tiny RCU grace periods

On Mon, Mar 22, 2021 at 08:47:44AM -0700, Paul E. McKenney wrote:
> On Sun, Mar 21, 2021 at 11:28:55PM +0100, Frederic Weisbecker wrote:
> > On Wed, Mar 03, 2021 at 04:26:31PM -0800, paulmck@...nel.org wrote:
> > > From: "Paul E. McKenney" <paulmck@...nel.org>
> > > 
> > > There is a need for a non-blocking polling interface for RCU grace
> > > periods, so this commit supplies start_poll_synchronize_rcu() and
> > > poll_state_synchronize_rcu() for this purpose.  Note that the existing
> > > get_state_synchronize_rcu() may be used if future grace periods are
> > > inevitable (perhaps due to a later call_rcu() invocation).  The new
> > > start_poll_synchronize_rcu() is to be used if future grace periods
> > > might not otherwise happen.  Finally, poll_state_synchronize_rcu()
> > > provides a lockless check for a grace period having elapsed since
> > > the corresponding call to either of the get_state_synchronize_rcu()
> > > or start_poll_synchronize_rcu().
> > > 
> > > As with get_state_synchronize_rcu(), the return value from either
> > > get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
> > > to a later call to either poll_state_synchronize_rcu() or the existing
> > > (might_sleep) cond_synchronize_rcu().
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > ---
> > >  include/linux/rcutiny.h | 11 ++++++-----
> > >  kernel/rcu/tiny.c       | 40 ++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 46 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > > index 2a97334..69108cf4 100644
> > > --- a/include/linux/rcutiny.h
> > > +++ b/include/linux/rcutiny.h
> > > @@ -17,14 +17,15 @@
> > >  /* Never flag non-existent other CPUs! */
> > >  static inline bool rcu_eqs_special_set(int cpu) { return false; }
> > >  
> > > -static inline unsigned long get_state_synchronize_rcu(void)
> > > -{
> > > -	return 0;
> > > -}
> > > +unsigned long get_state_synchronize_rcu(void);
> > > +unsigned long start_poll_synchronize_rcu(void);
> > > +bool poll_state_synchronize_rcu(unsigned long oldstate);
> > >  
> > >  static inline void cond_synchronize_rcu(unsigned long oldstate)
> > >  {
> > > -	might_sleep();
> > > +	if (poll_state_synchronize_rcu(oldstate))
> > > +		return;
> > > +	synchronize_rcu();
> > 
> > Perhaps cond_synchronize_rcu() could stay as it was. If it might
> > call synchronize_rcu() then it inherits its constraint to be
> > called from a quiescent state.
> 
> As in leave the might_sleep()?  How about something like this?
> 
> static inline void cond_synchronize_rcu(unsigned long oldstate)
> {
> 	if (!poll_state_synchronize_rcu(oldstate))
> 		synchronize_rcu();
> 	else
> 		might_sleep();
> }
> 
> One advantage of this is that the Tiny and Tree implementations
> become identical and can then be consolidated.
> 
> Or did I miss your point?

But poll_state_synchronize_rcu() checks that the gp_num has changed,
which is not needed for cond_synchronize_rcu() since this it is
only allowed to be called from a QS.

> 
> 						Thanx, Paul