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Re: [PATCH 1/3] Add ratelimited printk for different alert levels

To: Joe Perches <joe@xxxxxxxxxxx>
Subject: Re: [PATCH 1/3] Add ratelimited printk for different alert levels
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 13 Sep 2012 10:51:12 +1000
Cc: raghu.prabhu13@xxxxxxxxx, xfs@xxxxxxxxxxx, Raghavendra D Prabhu <rprabhu@xxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, Alex Elder <elder@xxxxxxxxxx>, open list <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <1347420159.2456.15.camel@joe2Laptop>
References: <cover.1347396641.git.rprabhu@xxxxxxxxxxx> <ef779779fdfb84c7c1216b374216246d4de83233.1347396641.git.rprabhu@xxxxxxxxxxx> <1347420159.2456.15.camel@joe2Laptop>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Sep 11, 2012 at 08:22:39PM -0700, Joe Perches wrote:
> On Wed, 2012-09-12 at 03:43 +0530, raghu.prabhu13@xxxxxxxxx wrote:
> > Ratelimited printk will be useful in printing xfs messages which are 
> > otherwise
> > not required to be printed always due to their high rate (to prevent kernel 
> > ring
> > buffer from overflowing), while at the same time required to be printed.
> []
> > diff --git a/fs/xfs/xfs_message.h b/fs/xfs/xfs_message.h
> []
> > @@ -30,6 +32,32 @@ void xfs_debug(const struct xfs_mount *mp, const char 
> > *fmt, ...)
> >  }
> >  #endif
> >  
> > +#define xfs_printk_ratelimited(xfs_printk, dev, fmt, ...)          \
> > +do {                                                                       
> > \
> > +   static DEFINE_RATELIMIT_STATE(_rs,                              \
> > +                                 DEFAULT_RATELIMIT_INTERVAL,       \
> > +                                 DEFAULT_RATELIMIT_BURST);         \
> > +   if (__ratelimit(&_rs))                                          \
> > +           xfs_printk(dev, fmt, ##__VA_ARGS__);                    \
> > +} while (0)
> 
> It might be better to use an xfs singleton RATELIMIT_STATE
> 
> DEFINE_RATELIMIT_STATE(xfs_rs);
> ...
> #define xfs_printk_ratelimited(xfs_printk, dev, fmt, ...)             \
> do {                                                                  \
>       if (__ratelimit(&xfs_rs))                                       \
>               xfs_printk(dev, fmt, ##__VA_ARGS__);                    \
> } while (0)

Which would then result in ratelimiting dropping potentially
important, unique messages. I think it's much better to guarantee
ratelimited messages get emitted at least once, especially as there
is the potential for multiple filesystems to emit messages
simultaneously.

I think per-location rate limiting is fine for the current usage -
ratelimiting is not widespread so there isn't a massive increase in
size as a result of this. If we do start to use ratelimiting in lots
of places in XFS, then we might have to revisit this, but it's OK
for now.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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