From: Uladzislau Rezki <urezki@gmail.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Matthew Wilcox <willy@infradead.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator
Date: Thu, 16 Jul 2020 16:37:14 +0200 [thread overview]
Message-ID: <20200716143714.GA30965@pc636> (raw)
In-Reply-To: <20200716133647.GA242690@google.com>
On Thu, Jul 16, 2020 at 09:36:47AM -0400, Joel Fernandes wrote:
> On Thu, Jul 16, 2020 at 11:19:13AM +0200, Uladzislau Rezki wrote:
> > On Wed, Jul 15, 2020 at 07:13:33PM -0400, Joel Fernandes wrote:
> > > On Wed, Jul 15, 2020 at 2:56 PM Sebastian Andrzej Siewior
> > > <bigeasy@linutronix.de> wrote:
> > > >
> > > > On 2020-07-15 20:35:37 [+0200], Uladzislau Rezki (Sony) wrote:
> > > > > @@ -3306,6 +3307,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > > > > if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > > > > return false;
> > > > >
> > > > > + preempt_disable();
> > > > > + krc_this_cpu_unlock(*krcp, *flags);
> > > >
> > > > Now you enter memory allocator with disabled preemption. This isn't any
> > > > better but we don't have a warning for this yet.
> > > > What happened to the part where I asked for a spinlock_t?
> > >
> > > Ulad,
> > > Wouldn't the replacing of preempt_disable() with migrate_disable()
> > > above resolve Sebastian's issue?
> > >
> > This for regular kernel only. That means that migrate_disable() is
> > equal to preempt_disable(). So, no difference.
>
> But this will force preempt_disable() context into the low-level page
> allocator on -RT kernels which I believe is not what Sebastian wants. The
> whole reason why the spinlock vs raw-spinlock ordering matters is, because on
> RT, the spinlock is sleeping. So if you have:
>
> raw_spin_lock(..);
> spin_lock(..); <-- can sleep on RT, so Sleep while atomic (SWA) violation.
>
> That's the main reason you are dropping the lock before calling the
> allocator.
>
No. Please read the commit message of this patch. This is for regular kernel.
You did a patch:
<snip>
if (IS_ENABLED(CONFIG_PREEMPT_RT))
return false;
<snip>
--
Vlad Rezki
next prev parent reply other threads:[~2020-07-16 14:37 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-15 18:35 [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator Uladzislau Rezki (Sony)
2020-07-15 18:56 ` Sebastian Andrzej Siewior
2020-07-15 19:02 ` Uladzislau Rezki
2020-07-15 19:32 ` Sebastian Andrzej Siewior
2020-07-15 19:36 ` Uladzislau Rezki
2020-07-15 22:14 ` Paul E. McKenney
2020-07-16 14:14 ` Sebastian Andrzej Siewior
2020-07-16 15:20 ` Paul E. McKenney
2020-07-16 15:36 ` Sebastian Andrzej Siewior
2020-07-16 16:36 ` Paul E. McKenney
2020-07-15 23:13 ` Joel Fernandes
2020-07-16 9:19 ` Uladzislau Rezki
2020-07-16 13:36 ` Joel Fernandes
2020-07-16 14:37 ` Uladzislau Rezki [this message]
2020-07-16 18:27 ` Joel Fernandes
2020-07-16 19:03 ` Uladzislau Rezki
2020-07-16 14:25 ` Sebastian Andrzej Siewior
2020-07-16 14:47 ` Uladzislau Rezki
2020-07-16 15:04 ` Sebastian Andrzej Siewior
2020-07-16 15:34 ` Uladzislau Rezki
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