linux-kernel - Re: [PATCH v1 4/5] kvfree_rcu: Refactor kfree_rcu

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Date:   Tue, 4 May 2021 15:46:18 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH v1 4/5] kvfree_rcu: Refactor kfree_rcu_monitor() function

On Mon, May 03, 2021 at 03:52:13PM -0700, Paul E. McKenney wrote:
> On Mon, May 03, 2021 at 08:12:14PM +0200, Uladzislau Rezki wrote:
> > Hello, Paul.
> > 
> > > Rearm the monitor work directly from its own function that
> > > is kfree_rcu_monitor(). So this patch puts the invocation
> > > timing control in one place.
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > ---
> > >  kernel/rcu/tree.c | 35 +++++++++++++++++++++--------------
> > >  1 file changed, 21 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index e44d6f8c56f0..229e909ad437 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3415,37 +3415,44 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
> > >  	return !repeat;
> > >  }
> > >  
> > > -static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
> > > -					  unsigned long flags)
> > > +/*
> > > + * This function queues a new batch. If success or nothing to
> > > + * drain it returns 1. Otherwise 0 is returned indicating that
> > > + * a reclaim kthread has not processed a previous batch.
> > > + */
> > > +static inline int kfree_rcu_drain(struct kfree_rcu_cpu *krcp)
> > >  {
> > > +	unsigned long flags;
> > > +	int ret;
> > > +
> > > +	raw_spin_lock_irqsave(&krcp->lock, flags);
> > > +
> > >  	// Attempt to start a new batch.
> > > -	if (queue_kfree_rcu_work(krcp)) {
> > > +	ret = queue_kfree_rcu_work(krcp);
> > > +	if (ret)
> > >  		// Success! Our job is done here.
> > >  		krcp->monitor_todo = false;
> > > -		raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > > -		return;
> > > -	}
> > >  
> > >  	// Previous RCU batch still in progress, try again later.
> > > -	schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> > >  	raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > > +	return ret;
> > >  }
> > >  
> > >  /*
> > >   * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
> > > - * It invokes kfree_rcu_drain_unlock() to attempt to start another batch.
> > > + * It invokes kfree_rcu_drain() to attempt to start another batch.
> > >   */
> > >  static void kfree_rcu_monitor(struct work_struct *work)
> > >  {
> > > -	unsigned long flags;
> > >  	struct kfree_rcu_cpu *krcp = container_of(work, struct kfree_rcu_cpu,
> > >  						 monitor_work.work);
> > >  
> > > -	raw_spin_lock_irqsave(&krcp->lock, flags);
> > > -	if (krcp->monitor_todo)
> > > -		kfree_rcu_drain_unlock(krcp, flags);
> > > -	else
> > > -		raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > > +	if (kfree_rcu_drain(krcp))
> > > +		return;
> > > +
> > > +	// Not success. A previous batch is still in progress.
> > > +	// Rearm a work to repeat an attempt of starting another batch.
> > > +	schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> > >  }
> > >  
> > >  static enum hrtimer_restart
> > > -- 
> > > 2.20.1
> > > 
> > 
> > Please see below a v2 of this patch. The main difference between v1
> > is that, the monitor work now is open-coded, thus some extra inline
> > functions were eliminated:
> > 
> > >From 7d153a640a4f791cbfd9b546e32f90fb2c60c480 Mon Sep 17 00:00:00 2001
> > From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
> > Date: Wed, 21 Apr 2021 13:22:52 +0200
> > Subject: [PATCH v2] kvfree_rcu: Refactor kfree_rcu_monitor()
> > 
> > Currently we have three functions which depend on each other.
> > Two of them are quite tiny and the last one where the most
> > work is done. All of them are related to queuing RCU batches
> > to reclaim objects after a GP.
> > 
> > 1. kfree_rcu_monitor(). It consist of few lines. It acquires
> >    the spin-lock and calls "drain" function.
> > 
> > 2. kfree_rcu_drain_unlock(). It also consists of few lines of
> >    code. It calls a func. to queue the batch. If not success
> >    rearm the monitor work to repeat an attempt one more time.
> > 
> > 3. queue_kfree_rcu_work(). Main core part is implemented here.
> >    In short, it attempts to start a new batch to free objects
> >    after a GP.
> > 
> > Since there are no external users of [2] and [3] functions we
> > can eliminate both by moving all logic directly into [1]. That
> > makes the kfree_rcu_monitor() to be open-coded what is easier
> > to follow thus becomes less complicated.
> > 
> > Apart of that, replace comments which start with "/*" to "//"
> > format to make it unified across the file.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> 
> Queued and pushed as shown below.  Nice diffstat!  ;-)
> 
After such refactoring everything seems has been settled :)

Thanks.

--
Vlad Rezki