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

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Message-ID: <20191007173644.hiiukrl2xryziro3@linutronix.de>
Date:   Mon, 7 Oct 2019 19:36:44 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Daniel Wagner <dwagner@...e.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 2019-10-07 18:56:11 [+0200], Uladzislau Rezki wrote:
> 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. 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.

What is to be expected on average? Is the lock acquired and then
released again because the slot is empty and memory needs to be
allocated or can it be assumed that this hardly happens? 

Sebastian