Hi Petr, thanks for review,
On Wed, 2018-08-15 at 17:10 +0200, Petr Mladek wrote:
On Tue 2018-07-03 23:56:28, Dmitry Safonov wrote:
Currently ratelimit_state is protected with spin_lock. If the .lock is taken at the moment of ___ratelimit() call, the message is suppressed to make ratelimiting robust.
That results in the following issue issue: CPU0 CPU1
printk_ratelimit() printk_ratelimit() | | try_spin_lock() try_spin_lock() | | time_is_before_jiffies() return 0; // suppress
So, concurrent call of ___ratelimit() results in silently suppressing one of the messages, regardless if the limit is reached or not. And rc->missed is not increased in such case so the issue is covered from user.
Convert ratelimiting to use atomic counters and drop spin_lock.
Note: That might be unexpected, but with the first interval of messages storm one can print up to burst*2 messages. So, it doesn't guarantee that in *any* interval amount of messages is lesser than burst. But, that differs lightly from previous behavior where one can start burst=5 interval and print 4 messages on the last milliseconds of interval + new 5 messages from new interval (totally 9 messages in lesser than interval value):
I am still confused by this paragraph. Does this patch change the behavior? What is the original and what is the new one, please?
Yeah, I could be a bit cleaner about the change. Originally more than `burst` messages could be printed if the previous period hasn't ended:
msg0 msg1-msg4 msg0-msg4 | | | | | | |--o---------------------o-|-----o--------------------|--> (t) <-------> Lesser than burst
But now, because I check: + if (atomic_add_unless(&rs->printed, 1, rs->burst)) + return 1;
*before* checking the end of interval, the maximum number of messages in the peak will be the same, burst*2 (burst*2 - 1 in original). Why do I check it before the end of interval? I thought it would be a nice optimization, making atomic_add_unless() the only called function on the fast-path (when we haven't hit the limit yet). If you want, I can move it into a separate patch..
Dropped dev/random patch since v1 version: lkml.kernel.org/r/20180510125211.12583-1-dima@arista.com
Dropped `name' in as it's unused in RATELIMIT_STATE_INIT()
diff --git a/lib/ratelimit.c b/lib/ratelimit.c index d01f47135239..d9b749d40108 100644 --- a/lib/ratelimit.c +++ b/lib/ratelimit.c @@ -13,6 +13,18 @@ #include <linux/jiffies.h> #include <linux/export.h>
+static void ratelimit_end_interval(struct ratelimit_state *rs, const char *func) +{
- rs->begin = jiffies;
- if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE)) {
unsigned int missed = atomic_xchg(&rs->missed, 0);if (missed)pr_warn("%s: %u callbacks suppressed\n",func, missed);
- }
+}
/*
- __ratelimit - rate limiting
- @rs: ratelimit_state data
@@ -27,45 +39,30 @@ */ int ___ratelimit(struct ratelimit_state *rs, const char *func) {
unsigned long flags;
int ret;
if (!rs->interval) return 1;
/*
* If we contend on this state's lock then almost* by definition we are too busy to print a message,* in addition to the one that will be printed by* the entity that is holding the lock already:*/if (!raw_spin_trylock_irqsave(&rs->lock, flags))
- if (unlikely(!rs->burst)) {
atomic_add_unless(&rs->missed, 1, -1);if (time_is_before_jiffies(rs->begin + rs-interval))
ratelimit_end_interval(rs, func);This is racy. time_is_before_jiffies() might be valid on two CPUs in parallel. They would both call ratelimit_end_interval(). This is not longer atomic context. Therefore one might get scheduled and set rs->begin = jiffies seconds later. I am sure that there might be more crazy scenarios.
That's the case with rs->burst == 0. So, in this situation all the messages will be suppressed. And the only reason to call ratelimit_end_interval() is to update the start time and number of messages not printed. I didn't want to add any complexion for this case - the worst will be the count of messages suppressed will be imprecise for rs->burst == 0 case. Not a big deal, huh?
- return 0;
- }
- if (!rs->begin)
rs->begin = jiffies;
if (atomic_add_unless(&rs->printed, 1, rs->burst))
return 1;if (time_is_before_jiffies(rs->begin + rs->interval)) {
if (rs->missed) {if (!(rs->flags &RATELIMIT_MSG_ON_RELEASE)) {
printk_deferred(KERN_WARNING"%s: %d callbackssuppressed\n",
func, rs->missed);rs->missed = 0;}}rs->begin = jiffies;rs->printed = 0;- }
- if (rs->burst && rs->burst > rs->printed) {
rs->printed++;ret = 1;- } else {
rs->missed++;ret = 0;
if (atomic_cmpxchg(&rs->printed, rs->burst, 0))ratelimit_end_interval(rs, func);
raw_spin_unlock_irqrestore(&rs->lock, flags);
return ret;
- if (atomic_add_unless(&rs->printed, 1, rs->burst))
return 1;The entire logic is complicated and hard to understand. Especially calling ratelimit_end_interval() and atomic_add_unless(&rs->printed) twice.
Probably, I've described the reasons above.. I may add a comment or drop the first atomic_add_unless(), but I would prefer still keep it there, having only one check for fast-path.
- atomic_add_unless(&rs->missed, 1, -1);
- return 0;
}
I wonder if the following code would do the job (not even compile tested!):
static void ratelimit_end_interval(struct ratelimit_state *rs, const char *func) { rs->begin = jiffies;
if (!(rs->flags & RATELIMIT_MSG_ON_RELEASE)) { unsigned int missed = atomic_xchg(&rs->missed, 0);
if (missed) pr_warn("%s: %u callbacks suppressed\n", func,missed); }
atomic_xchg(&rs->printed, 0); }
/*
- __ratelimit - rate limiting
- @rs: ratelimit_state data
- @func: name of calling function
- This enforces a rate limit: not more than @rs->burst callbacks
- in every @rs->interval
- RETURNS:
- 0 means callbacks will be suppressed.
- 1 means go ahead and do it.
*/ int ___ratelimit(struct ratelimit_state *rs, const char *func) { unsigned long begin, old_begin = rs->begin;
if (!rs->interval) return 1;
if (time_is_before_jiffies(rs->begin + rs->interval) && cmpxchg(&rs->begin, begin, begin + rs->interval) == begin)
Probably, cmpxchg(&rs->begin, begin, jiffies)? Otherwise hitting it later will be buggy. Also `begin` might be uninitialized? (if cmpxchg() fails)
{ ratelimit_end_interval(rs, func); }
if (atomic_add_unless(&rs->printed, 1, rs->burst)) return 1;
atomic_add_unless(&rs->missed, 1, -1); return 0; } EXPORT_SYMBOL(___ratelimit);
The main logic is the same as in the original code. Only one CPU is able to reset the interval and counters (thanks to cmpxchg). Every caller increases either "printed" or "missed" counter.
Do we care about fast-path argument I did above?