linux-kernel - Re: [PATCH 3/3] vdso: preallocate new vmas

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Date:	Thu, 24 Oct 2013 17:55:41 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	walken@...gle.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, aswin@...com,
	linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Richard Kuo <rkuo@...eaurora.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/3] vdso: preallocate new vmas

On Wed, 2013-10-23 at 02:53 -0700, walken@...gle.com wrote:
> On Sun, Oct 20, 2013 at 08:26:15PM -0700, Davidlohr Bueso wrote:
> > From: Davidlohr Bueso <davidlohr@...com>
> > Subject: [PATCH v2 3/3] vdso: preallocate new vmas
> > 
> > With the exception of um and tile, architectures that use
> > the install_special_mapping() function, when setting up a
> > new vma at program startup, do so with the mmap_sem lock
> > held for writing. Unless there's an error, this process
> > ends up allocating a new vma through kmem_cache_zalloc,
> > and inserting it in the task's address space.
> > 
> > This patch moves the vma's space allocation outside of
> > install_special_mapping(), and leaves the callers to do so
> > explicitly, without depending on mmap_sem. The same goes for
> > freeing: if the new vma isn't used (and thus the process fails
> > at some point), it's caller's responsibility to free it -
> > currently this is done inside install_special_mapping.
> > 
> > Furthermore, uprobes behaves exactly the same and thus now the
> > xol_add_vma() function also preallocates the new vma.
> > 
> > While the changes to x86 vdso handling have been tested on both
> > large and small 64-bit systems, the rest of the architectures
> > are totally *untested*. Note that all changes are quite similar
> > from architecture to architecture.
> > 
> > Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> > Cc: Russell King <linux@....linux.org.uk>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Richard Kuo <rkuo@...eaurora.org>
> > Cc: Ralf Baechle <ralf@...ux-mips.org>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> > Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> > Cc: Paul Mundt <lethal@...ux-sh.org>
> > Cc: Chris Metcalf <cmetcalf@...era.com>
> > Cc: Jeff Dike <jdike@...toit.com>
> > Cc: Richard Weinberger <richard@....at>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> > v2:
> > - Simplify install_special_mapping interface (Linus Torvalds)
> > - Fix return for uml_setup_stubs when mem allocation fails (Richard Weinberger)
> 
> I'm still confused as to why you're seeing any gains with this
> one. This code runs during exec when mm isn't shared with any other
> threads yet, so why does it matter how long the mmap_sem is held since
> nobody else can contend on it ? (well, except for accesses from
> /fs/proc/base.c, but I don't see why these would matter in your
> benchmarks either).

Yeah, that's why I dropped the performance numbers from the changelog in
v2, of course any differences are within the noise range. When I did the
initial runs I was scratching my head as to why I was seeing benefits,
but it was most likely a matter of clock frequency differences, and I no
longer see such boosts.

Thanks,
Davidlohr


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