linux-kernel - Re: [PATCH 1/2] rcu/kvfree: Remove useless monitor

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Message-ID: <20220604160353.GV1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Sat, 4 Jun 2022 09:03:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.iitr10@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 1/2] rcu/kvfree: Remove useless monitor_todo flag

On Thu, Jun 02, 2022 at 11:43:39PM +0000, Joel Fernandes wrote:
> On Thu, Jun 02, 2022 at 10:06:43AM +0200, Uladzislau Rezki (Sony) wrote:
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > monitor_todo is not needed as the work struct already tracks
> > if work is pending. Just use that to know if work is pending
> > using schedule_delayed_work() helper.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/tree.c | 33 ++++++++++++++++-----------------
> >  1 file changed, 16 insertions(+), 17 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 222d59299a2a..fd16c0b46d9e 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3295,7 +3295,6 @@ struct kfree_rcu_cpu_work {
> >   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> >   * @lock: Synchronize access to this structure
> >   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> > - * @monitor_todo: Tracks whether a @monitor_work delayed work is pending
> >   * @initialized: The @rcu_work fields have been initialized
> >   * @count: Number of objects for which GP not started
> >   * @bkvcache:
> > @@ -3320,7 +3319,6 @@ struct kfree_rcu_cpu {
> >  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> >  	raw_spinlock_t lock;
> >  	struct delayed_work monitor_work;
> > -	bool monitor_todo;
> >  	bool initialized;
> >  	int count;
> >  
> > @@ -3500,6 +3498,18 @@ static void kfree_rcu_work(struct work_struct *work)
> >  	}
> >  }
> >  
> > +static bool
> > +need_offload_krc(struct kfree_rcu_cpu *krcp)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < FREE_N_CHANNELS; i++)
> > +		if (krcp->bkvhead[i])
> > +			return true;
> > +
> > +	return !!krcp->head;
> > +}
> 
> Thanks for modifying my original patch to do this, and thanks for giving me
> the attribution for the patch. This function is a nice addition.
> 
> For the patch in its entirety:
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

I pulled this one in for testing and further review, thank you both!

Given the description, I reversed the order of the Signed-off-by
tags to indicate that the patch came through Uladzislau from Joel.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> 
> > +
> >  /*
> >   * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
> >   */
> > @@ -3556,9 +3566,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
> >  	// of the channels that is still busy we should rearm the
> >  	// work to repeat an attempt. Because previous batches are
> >  	// still in progress.
> > -	if (!krcp->bkvhead[0] && !krcp->bkvhead[1] && !krcp->head)
> > -		krcp->monitor_todo = false;
> > -	else
> > +	if (need_offload_krc(krcp))
> >  		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> >  
> >  	raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > @@ -3746,11 +3754,8 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  	WRITE_ONCE(krcp->count, krcp->count + 1);
> >  
> >  	// Set timer to drain after KFREE_DRAIN_JIFFIES.
> > -	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> > -	    !krcp->monitor_todo) {
> > -		krcp->monitor_todo = true;
> > +	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
> >  		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> > -	}
> >  
> >  unlock_return:
> >  	krc_this_cpu_unlock(krcp, flags);
> > @@ -3825,14 +3830,8 @@ void __init kfree_rcu_scheduler_running(void)
> >  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> >  
> >  		raw_spin_lock_irqsave(&krcp->lock, flags);
> > -		if ((!krcp->bkvhead[0] && !krcp->bkvhead[1] && !krcp->head) ||
> > -				krcp->monitor_todo) {
> > -			raw_spin_unlock_irqrestore(&krcp->lock, flags);
> > -			continue;
> > -		}
> > -		krcp->monitor_todo = true;
> > -		schedule_delayed_work_on(cpu, &krcp->monitor_work,
> > -					 KFREE_DRAIN_JIFFIES);
> > +		if (need_offload_krc(krcp))
> > +			schedule_delayed_work_on(cpu, &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> >  		raw_spin_unlock_irqrestore(&krcp->lock, flags);
> >  	}
> >  }
> > -- 
> > 2.30.2
> >