From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
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 17:04:14 +0200 [thread overview]
Message-ID: <20200716150414.iqpyby6nrww4zbyk@linutronix.de> (raw)
In-Reply-To: <20200716144728.GA31046@pc636>
On 2020-07-16 16:47:28 [+0200], Uladzislau Rezki wrote:
> On Thu, Jul 16, 2020 at 04:25:37PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-07-16 11:19:13 [+0200], Uladzislau Rezki wrote:
> > > Sebastian, could you please confirm that if that patch that is in
> > > question fixes it?
> > >
> > > It would be appreciated!
> >
> > So that preempt disable should in terms any warnings. However I don't
> > think that it is strictly needed and from scheduling point of view you
> > forbid a CPU migration which might be good otherwise.
> >
> Please elaborate your point regarding "i do not think it is strictly needed".
>
> Actually i can rework the patch to remove even such preempt_enable/disable
> to stay on the same CPU, but i do not see the point of doing it.
>
> Do you see the point?
You disable preemption for what reason? It is not documented, it is not
obvious - why is it required?
> As for scheduling point of view. Well, there are many places when there
> is a demand in memory or pages from atomic context. Also, getting a page
> is not considered as a hot path in the kfree_rcu().
If you disable preemption than you assume that you wouldn't be atomic
otherwise. You say that at this point it is not a hot path so if this is
not *that* important why not allow preemption and allow the schedule to
place you somewhere else if the scheduler decides that it is a good idea.
> > Also if interrupts and everything is enabled then someone else might
> > invoke kfree_rcu() from BH context for instance.
> >
> And what? What is a problem here, please elaborate if you see any
> issues.
That the kfree_rcu() caller from BH context will end up here as well,
asking for a page.
Sebastian
next prev parent reply other threads:[~2020-07-16 15:04 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
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 [this message]
2020-07-16 15:34 ` Uladzislau Rezki
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