linux-kernel - Re: [PATCH 3/3] kvfree_rcu: use migrate

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Date:   Wed, 20 Jan 2021 13:42:53 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        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>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 3/3] kvfree_rcu: use migrate_disable/enable()

On Wed, Jan 20, 2021 at 08:45:54PM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-01-20 17:21:48 [+0100], Uladzislau Rezki (Sony) wrote:
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3489,10 +3489,12 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
> >  			(*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> >  		bnode = get_cached_bnode(*krcp);
> >  		if (!bnode && can_alloc) {
> > +			migrate_disable();
> >  			krc_this_cpu_unlock(*krcp, *flags);
> 
> Why is krc_this_cpu_unlock() defined as
> | static inline void
> | krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
> | {
> |         raw_spin_unlock(&krcp->lock);
> |         local_irq_restore(flags);
> | }
> 
> instead of raw_spin_unlock_irqrestore()?
> Should something with the locked section trigger a scheduling event by
> setting TIF_NEED_RESCHED then there will be no scheduling event on
> unlock. It will be delayed until a later "random" preempt_enable().
> 
> raw_spin_unlock_irqrestore() will reschedule if the flag is set,
> local_irq_restore() will not.

Good catch, thank you!  This one is already in mainline, so I queued
the following patch.

							Thanx, Paul

------------------------------------------------------------------------

commit 6c1d51e012c5b474cda77d4fa644d76e041c1e05
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Wed Jan 20 13:38:08 2021 -0800

    kvfree_rcu: Make krc_this_cpu_unlock() use raw_spin_unlock_irqrestore()
    
    The krc_this_cpu_unlock() function does a raw_spin_unlock() immediately
    followed by a local_irq_restore().  This commit saves a line of code by
    merging them into a raw_spin_unlock_irqrestore().  This transformation
    also reduces scheduling latency because raw_spin_unlock_irqrestore()
    responds immediately to a reschedule request.  In contrast,
    local_irq_restore() does a scheduling-oblivious enabling of interrupts.
    
    Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cad3607..e7a226a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3201,8 +3201,7 @@ krc_this_cpu_lock(unsigned long *flags)
 static inline void
 krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
 {
-	raw_spin_unlock(&krcp->lock);
-	local_irq_restore(flags);
+	raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
 static inline struct kvfree_rcu_bulk_data *