Re: [PATCHv3] lib/ratelimit: Lockless ratelimiting

[Dmitry Safonov <dima@arista.com>]

On Fri, 2018-07-20 at 17:09 +0200, Petr Mladek wrote:
> > > On Tue, 2018-07-03 at 23:56 +0100, 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):
> > > > 
> > > >    msg0              msg1-msg4 msg0-msg4
> > > >     |                     |       |
> > > >     |                     |       |
> > > >  |--o---------------------o-|-----o--------------------|--> (t)
> > > >                           <------->
> > > >                        Lesser than burst
> > > > 
> 
> I am a bit confused. Does this patch fix two problems?
> 
>    + Lost rc->missed update when try_spin_lock() fails
>    + printing up to burst*2 messages in a give interval

What I tried to solve here (maybe I could better point it in the commit
message): is the situation where ratelimit::burst haven't been hit yet
and there are calls for __ratelimit() from different CPUs.
So, neither of the messages should be suppressed, but as the spinlock
is taken already - one of them is dropped and even without updating
missed counter.

> It would make the review much easier if you split it into more
> patches and fix the problems separately.

Hmm, not really sure: the patch changes spinlock to atomics and I'm not
sure it make much sense to add atomics before removing the spinlock..

> Otherwise, it looks promissing...
> 
> Best Regards,
> Petr
> 
> PS: I have vacation the following two weeks. Anyway, please, CC me
> if you send any new version.

Surely, thanks for your attention to the patch (and time).

-- 
Thanks,
             Dmitry
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