| Message ID | 1438705579-18461-1-git-send-email-Jason@zx2c4.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
From: "Jason A. Donenfeld" <Jason@zx2c4.com> Date: Tue, 4 Aug 2015 18:26:19 +0200 > 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. > > Also, though not strictly neccessary, it guards the call with an if (0) > so that all evaluation of any arguments are sure to be compiled out. > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Applied to net-next, thanks. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/include/linux/net.h b/include/linux/net.h index 04aa068..049d4b0 100644 --- a/include/linux/net.h +++ b/include/linux/net.h @@ -239,8 +239,16 @@ do { \ net_ratelimited_function(pr_warn, fmt, ##__VA_ARGS__) #define net_info_ratelimited(fmt, ...) \ net_ratelimited_function(pr_info, fmt, ##__VA_ARGS__) +#if defined(DEBUG) #define net_dbg_ratelimited(fmt, ...) \ net_ratelimited_function(pr_debug, fmt, ##__VA_ARGS__) +#else +#define net_dbg_ratelimited(fmt, ...) \ + do { \ + if (0) \ + no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__); \ + } while (0) +#endif bool __net_get_random_once(void *buf, int nbytes, bool *done, struct static_key *done_key);
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. Also, though not strictly neccessary, it guards the call with an if (0) so that all evaluation of any arguments are sure to be compiled out. Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- include/linux/net.h | 8 ++++++++ 1 file changed, 8 insertions(+)