linux-kernel - Re: [PATCH] 9p: remove CONFIG_NET_9P

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Date:	Thu, 11 Aug 2011 12:05:08 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Eric Van Hensbergen <ericvh@...il.com>,
	Alex Ray <alexjray.ncsu@...il.com>,
	v9fs-developer@...ts.sourceforge.net,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alex Ray <ajray@...u.edu>
Subject: Re: [PATCH] 9p: remove CONFIG_NET_9P_DEBUG option

On Thu, 2011-08-11 at 21:24 +0530, Aneesh Kumar K.V wrote:
> On Wed, 10 Aug 2011 13:36:52 -0500, Eric Van Hensbergen <ericvh@...il.com> wrote:
> > On Wed, Aug 10, 2011 at 12:17 PM, Aneesh Kumar K.V
> > <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> > > On Wed, 10 Aug 2011 07:24:56 -0500, Eric Van Hensbergen <ericvh@...il.com> wrote:
> > >> On Wed, Aug 10, 2011 at 4:13 AM, Aneesh Kumar K.V
> > >> <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> > >> > On Mon,  1 Aug 2011 07:14:44 -0500, Alex Ray <alexjray.ncsu@...il.com> wrote:
> > >> >> Remove the CONFIG_NET_9P_DEBUG option, used to completely remove logging
> > >> >> functionality from v9fs.  Logging is (already) controlled with the
> > >> >> run-time debug= option, this gets rid of the compile-time option (which
> > >> >> was being misunderstood and misused).
> > >> >>
> > >> >> Signed-off-by: Alex Ray <ajray@...u.edu>
> > >> >
> > >> > I see this merged to for-next.  Do we know whether enabling debug always have a
> > >> > performance impact ?.
> > >> >
> > >>
> > >> No clue, but without any debug it makes it impossible for user's to
> > >> generate reasonable bug reports.  If I understand the tracepoint
> > >> collection facility correctly, it incurs exactly the same overhead as
> > >> a DPRINT when the debug mount option is set to 0 (although tracepoints
> > >> are much lower overhead when actually collecting).
> > >
> > > I was worried about overhead when we are not collecting any debug info.
> > >
> > 
> > I understand that.  But the overhead when not collecting is the
> > conditional branch.
> > According to Documentation/trace/tracepoints.txt this is the same for the
> > tracepoints:
> > 
> > "When a tracepoint is "off" it has no effect, except for adding a tiny
> > time penalty
> > (checking a condition for a branch) and space penalty (adding a few
> > bytes for the function call at the end of the instrumented function
> > and adds a data structure in a separate section)."
> > 
> > So, since DPRINT is essentially if(p9_debug_level & level) == level)
> > it should roughly amount to the same overhead, no?  I suppose we could
> > get fancy and and prefix it with an unlikely.
> > 
> 
> Is that true with jump label ? May be we should update tracepoints.txt ?

Correct, that should be updated. When jump labels are enabled, we have a
nop and an unconditional branch over the trace code, which when enable
will change the nop to call the trace code.

We do have a slight hit in icache, but nothing I have been able to
measure. But if we do, I could even modify it to become even less of an
impact (but will cause tracing to slow down, but we don't really care if
it speeds up non tracing case ;)

-- Steve


> 
> Upstream commit:
> bf5438fca2950b03c21ad868090cc1a8fcd49536
> 8f7b50c514206211cc282a4247f7b12f18dee674
> 
> -aneesh


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