linux-kernel - Re: [PATCH RT 5/5] allow preemption in slab_alloc_node and slab

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Date:	Fri, 14 Feb 2014 15:07:29 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	linux-rt-users@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Platschek <platschek@....tuwien.ac.at>
Subject: Re: [PATCH RT 5/5] allow preemption in slab_alloc_node and slab_free

* Nicholas Mc Guire | 2014-02-10 16:40:16 [+0100]:

>__slab_alloc is only called from slub.c:slab_alloc_node
>it runs with local irqs disabled so it can't be pushed off this CPU
>asynchronously, the preempt_disable/enable is thus not needed.
>Aside from that the later this_cpu_cmpxchg_double would catch such a 
>migration event anyay.

Not sure what to do with this one. You do write a longer chapter why it
is okay to drop the preemption disable section and that
this_cpu_cmpxchg_double() would catch it. And I didn't figure out so
far why need to keep preemption disabled while looking at c->tid but not
at c->page.
However, it seems that Christoph Lameter found it important to add a
note in the comment that this preemption disable here is important.
Looking at commit 7cccd80 ("slub: tid must be retrieved from the percpu
area of the current processor") it seems that Steven Rostedt run into
trouble and now we have that preemption_disable() here.

So if you really get better performance and you haven't seen anything
bad happen then you might want to check with Lameter & Rostedt about
your patch and getting it merged upstream.
The commit I mentioned is upstream since v3.11-rc1 and I can see it in
v3.8-RT tree so it looks serious.
I fail to see it in v3.2-RT, Steven, isn't this something we want there,
too?

Sebastian
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