linux-kernel - Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator

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Message-ID: <20200716143714.GA30965@pc636>
Date:   Thu, 16 Jul 2020 16:37:14 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page
 allocator

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@...utronix.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