linux-kernel - Re: [PATCH] [v3] warn on performance-impacting configs aka. TAINT

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Date:	Fri, 22 Aug 2014 11:12:40 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
	peterz@...radead.org, mingo@...hat.com, ak@...ux.intel.com,
	akpm@...ux-foundation.org, cl@...ux.com, penberg@...nel.org,
	linux-mm@...ck.org, kirill@...temov.name, lauraa@...eaurora.org
Subject: Re: [PATCH] [v3] warn on performance-impacting configs aka.
 TAINT_PERFORMANCE

On Fri, 2014-08-22 at 09:45 -0700, Dave Hansen wrote:
> On 08/22/2014 09:32 AM, Tim Chen wrote:
> >> > +#ifdef CONFIG_DEBUG_OBJECTS_FREE
> >> > +	"DEBUG_OBJECTS_FREE",
> >> > +#endif
> >> > +#ifdef CONFIG_DEBUG_KMEMLEAK
> >> > +	"DEBUG_KMEMLEAK",
> >> > +#endif
> >> > +#ifdef CONFIG_DEBUG_PAGEALLOC
> >> > +	"DEBUG_PAGEALLOC",
> > I think coverage profiling also impact performance.
> > So I sould also put CONFIG_GCOV_KERNEL in the list.
> 
> Would CONFIG_GCOV_PROFILE_ALL be the better one to check?  With plain
> GCOV_KERNEL, I don't think we will, by default, put the coverage
> information in any files and slow them down.

CONFIG_GCOV_PROFILE_ALL is definitely a no no regarding to
performance impact, which is mentioned in the gcov documentation.

I haven't tested this, but if profiling is turned on only for
a piece of code that is performance critical but not for
the whole kernel, in theory performance can still be impacted
with the overhead.  So I think it is safer to check
for CONFIG_GCOV_KERNEL, that has no reason to be turned on
for any workload that's performance critical.

Tim


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