linux-kernel - Re: [tip:tracing/urgent] tracing: Fix too large stack usage in do_one

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Date:	Fri, 21 Aug 2009 13:37:33 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-tip-commits@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Kyle McMartin <kyle@...artin.ca>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
	torvalds@...ux-foundation.org, catalin.marinas@....com,
	jens.axboe@...cle.com, fweisbec@...il.com, stable@...nel.org,
	srostedt@...hat.com, tglx@...utronix.de
Subject: Re: [tip:tracing/urgent] tracing: Fix too large stack usage in
 do_one_initcall()

On Fri, 2009-08-21 at 13:14 +0200, Ingo Molnar wrote:

> > There's a lot of fat functions on that stack trace, but
> > the largest of all is do_one_initcall(). This is due to
> > the boot trace entry variables being on the stack.
> > 
> > Fixing this is relatively easy, initcalls are fundamentally
> > serialized, so we can move the local variables to file scope.
> > 
> > Note that this large stack footprint was present for a
> > couple of months already - what pushed my system over
> > the edge was the addition of kmemleak to the call-chain:
> > 
> >   6)     3328      36   allocate_slab+0xb1/0x100
> >   7)     3292      36   new_slab+0x1c/0x160
> >   8)     3256      36   __slab_alloc+0x133/0x2b0
> >   9)     3220       4   kmem_cache_alloc+0x1bb/0x1d0
> >  10)     3216     108   create_object+0x28/0x250
> >  11)     3108      40   kmemleak_alloc+0x81/0xc0
> >  12)     3068      24   kmem_cache_alloc+0x162/0x1d0
> >  13)     3044      52   scsi_pool_alloc_command+0x29/0x70
> > 
> > This pushes the total to ~3800 bytes, only a tiny bit
> > more was needed to corrupt the on-kernel-stack thread_info.
> > 
> > The fix reduces the stack footprint from 572 bytes
> > to 28 bytes.
> 
> btw., it will just take two more features like kmemleak to trigger 
> hard to debug stack overflows again on 32-bit. We are right at the 
> edge and this situation is not really fixable in a reliable way 
> anymore.
> 
> So i think we should be more drastic and solve the real problem: we 
> should drop 4K stacks and 8K combo-stacks on 32-bit, and go 
> exclusively to 8K split stacks on 32-bit.
> 
> I.e. the stack size will be 'unified' too between 64-bit and 32-bit 
> to a certain degree: process stacks will be 8K on both 64-bit and 
> 32-bit x86, IRQ stacks will be separate. (on 64-bit we also have the 
> IST stacks for certain exceptions that further isolates things)
> 
> This will simplify the 32-bit situation quite a bit and removes a 
> contentious config option and makes the kernel more robust in 
> general. 8K combo stacks are not safe due to irq nesting and 4K 
> isolated stacks are not enough. 8K isolated stacks is the way to go.
> 
> Opinions?

I'm obviously all in favour of merging the i386 and x86_64 stack code.
Esp after having had to look at the i386 stuff recently.

Now I don't think that unifying all this requires the sizes to be the
same between them, because x86_64 typically has larger stack footprint
due to it being 64 bit. If we need to bump 32 bit stack sizes, then
we're likely to also need a bump in 64 bit as well at some point soon.


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