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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
