linux-kernel - Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect ne_fit_preload

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Message-ID: <20191007172259.7mdthvqua4wwyold@beryllium.lan>
Date:   Mon, 7 Oct 2019 19:22:59 +0200
From:   Daniel Wagner <dwagner@...e.de>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect
 ne_fit_preload_node

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.