linux-kernel - Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty

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Date:	Thu, 26 Sep 2013 07:31:47 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Fengguang Wu <fengguang.wu@...el.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Tejun Heo <tj@...nel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers
 into n_tty_data"

On 09/26/2013 03:33 AM, Andrew Morton wrote:
> On Tue, 17 Sep 2013 20:22:42 -0400 Peter Hurley <peter@...leysoftware.com> wrote:
>
>> Looking over vmalloc.c, the critical section footprint of the vmap_area_lock
>> could definitely be reduced (even nearly eliminated), but that's a project for
>> another day :)
>
> 20bafb3d23d10 ("n_tty: Move buffers into n_tty_data") switched a
> kmalloc (which is very fast) to a vmalloc (which is very slow) without
> so much as mentioning it in the changelog.  This should have been
> picked up at review, btw.
>
> Revert that part of the patch and the problem will be solved.
>
> If we are really really worried that a ~9k kmalloc might fail or will
> be slow, then implement a fallback to vmalloc() if kmalloc(GFP_NOWARN)
> failed.  This kinda sucks, but is practical, but really should only be
> done if necessary - ie, if problems with using plain old kmalloc are
> demonstrable.
>
> Or just revert all of 20bafb3d23d10 - it was supposed to be a small
> performance improvement but turned out to be a significant performance
> loss.  Therefore zap.

I have no particular objection to reverting the entire patch.

However, it's a mischaracterization to suggest that the reason is
because vmalloc() is very slow;  without reading /proc/meminfo,
there is no performance loss.

IOW, the lock contention this patch precipitated needs to get fixed
regardless.


Regards,
Peter Hurley
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