linux-kernel - Re: [PATCH] net_dbg_ratelimited: turn into no-op when !DEBUG

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Message-ID: <1438660677.10829.1.camel@perches.com>
Date:	Mon, 03 Aug 2015 20:57:57 -0700
From:	Joe Perches <joe@...ches.com>
To:	"Jason A. Donenfeld" <Jason@...c4.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net_dbg_ratelimited: turn into no-op when !DEBUG

On Tue, 2015-08-04 at 05:26 +0200, Jason A. Donenfeld wrote:
> The pr_debug family of functions turns into a no-op when -DDEBUG is not
> specified, opting instead to call "no_printk", which gets compiled to a
> no-op (but retains gcc's nice warnings about printf-style arguments).
> 
> The problem with net_dbg_ratelimited is that it is defined to be a
> variant of net_ratelimited_function, which expands to essentially:
> 
>     if (net_ratelimit())
>         pr_debug(fmt, ...);
> 
> When DEBUG is not defined, then this becomes,
> 
>     if (net_ratelimit())
>         ;
> 
> This seems benign, except it isn't. Firstly, there's the obvious
> overhead of calling net_ratelimit needlessly, which does quite some book
> keeping for the rate limiting. Given that the pr_debug and
> net_dbg_ratelimited family of functions are sprinkled liberally through
> performance critical code, with developers assuming they'll be compiled
> out to a no-op most of the time, we certainly do not want this needless
> book keeping. Secondly, and most visibly, even though no debug message
> is printed when DEBUG is not defined, if there is a flood of
> invocations, dmesg winds up peppered with messages such as
> "net_ratelimit: 320 callbacks suppressed". This is because our
> aforementioned net_ratelimit() function actually prints this text in
> some circumstances. It's especially odd to see this when there isn't any
> other accompanying debug message.
> 
> So, in sum, it doesn't make sense to have this function's current
> behavior, and instead it should match what every other debug family of
> functions in the kernel does with !DEBUG -- nothing.
> 
> This patch replaces calls to net_dbg_ratelimited when !DEBUG with
> no_printk, keeping with the idiom of all the other debug print helpers.

Makes sense, thanks Jason.



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