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 <[email protected]>
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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
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 *
