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From: Daniel Wagner <dwagner@suse.de>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-rt-users@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect ne_fit_preload_node
Date: Mon, 7 Oct 2019 19:22:59 +0200
Message-ID: <20191007172259.7mdthvqua4wwyold@beryllium.lan> (raw)
In-Reply-To: <20191007165611.GA26964@pc636>

On Mon, Oct 07, 2019 at 06:56:11PM +0200, Uladzislau Rezki wrote:
> On Mon, Oct 07, 2019 at 06:34:43PM +0200, Daniel Wagner wrote:
> > I supppose, one thing which would help in this discussion, is what do
> > you gain by using preempt_disable() instead of moving the lock up?
> > Do you have performance numbers which could justify the code?
> >
> Actually there is a high lock contention on vmap_area_lock, because it
> is still global. You can have a look at last slide:
> 
> https://linuxplumbersconf.org/event/4/contributions/547/attachments/287/479/Reworking_of_KVA_allocator_in_Linux_kernel.pdf
> 
> so this change will make it a bit higher.

Thanks! I suspected something like this :(

On the todo-list page you stating that vmap_area_lock could be
splitted and therefore reduce the contention. If you could avoid those
preempt_disable() tricks and just use plain spin_locks() to protect it
would be really helpful.

> From the other hand i agree
> that for rt it should be fixed, probably it could be done like:
> 
> ifdef PREEMPT_RT
>     migrate_disable()
> #else
>     preempt_disable()
> ...
> 
> but i am not sure it is good either.

I don't think this way to go. I guess Sebastian and Thomas have a
better idea how to address this for PREEMPT_RT.

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-03  9:09 Daniel Wagner
2019-10-03 11:55 ` Uladzislau Rezki
2019-10-04 15:37 ` Sebastian Andrzej Siewior
2019-10-04 16:20   ` Uladzislau Rezki
2019-10-04 16:30     ` Sebastian Andrzej Siewior
2019-10-04 17:04       ` Uladzislau Rezki
2019-10-04 17:45         ` Sebastian Andrzej Siewior
2019-10-07  8:30       ` Daniel Wagner
2019-10-07 10:56         ` Sebastian Andrzej Siewior
2019-10-07 16:23           ` Uladzislau Rezki
2019-10-07 16:34             ` Daniel Wagner
2019-10-07 16:56               ` Uladzislau Rezki
2019-10-07 17:22                 ` Daniel Wagner [this message]
2019-10-07 17:36                 ` Sebastian Andrzej Siewior
2019-10-07 21:44                   ` Uladzislau Rezki
2019-10-08 16:04                     ` Uladzislau Rezki
2019-10-09  6:05                       ` Daniel Wagner
2019-10-09  9:47                         ` Uladzislau Rezki
2019-10-07  8:27   ` Daniel Wagner

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