linux-kernel - Re: [PATCH v4 09/11] powerpc/perf: add support for the hv 24x7 interface

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Message-ID: <537E78AD.1040409@linux.vnet.ibm.com>
Date:	Thu, 22 May 2014 15:22:37 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Ian Munsie <imunsie@....ibm.com>
CC:	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	scottwood <scottwood@...escale.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v4 09/11] powerpc/perf: add support for the hv 24x7 interface

On 05/22/2014 01:19 AM, Ian Munsie wrote:
> Hi Cody,
>
> I just tried building this with gcc 4.5, which failed with the following
> warning (treated as an error):
>
> cc1: warnings being treated as errors
> arch/powerpc/perf/hv-24x7.c: In function 'single_24x7_request':
> arch/powerpc/perf/hv-24x7.c:346:1: error: the frame size of 8192 bytes is larger than 2048 bytes
> make[3]: *** [arch/powerpc/perf/hv-24x7.o] Error 1
> make[2]: *** [arch/powerpc/perf] Error 2
>
> My .config has CONFIG_FRAME_WARN=2048 (default on 64bit), but the
> alignment constraints in this function may require 8K on the stack -
> possibly a bit large?
>

Yep, it is a bit large. In other places in hv-24x7 that use similar 
firmware interfaces (with similar alignment requirements), I've used a 
kmem_cache (hv_page_cache). Testing out a patch that uses that here as well.

>
> Notably for some reason this warning no longer seems to trigger on gcc
> 4.8 (or at least somewhere between 4.5-4.8), though the assembly does
> still show it aligning the buffers.

That's a bit concerning (and might be why I didn't pick it up, using gcc 
4.9.0 over here). Looking at the gcc docs, it seems to indicate that 
alloca() and VLAs aren't counted for -Wframe-larger-than. Perhaps gcc 
decided to move locally defined structures with alignment requirements 
into that same bucket? (while size of the structures is statically 
determinable, the stack consumption due to alignment is [to some degree] 
variable).

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