[Lse-tech] Re: Lockmeter 1.4.10 problem

[Maneesh S. <ma...@in...>]

On Fri, Dec 14, 2001 at 10:11:25AM -0800, John Hawkes wrote:

> I'm guessing that you're looking at i386, which I believe means that the
> code that executes is found in arch/i386/lib/dec_and_lock.c, right?

Yes I am looking at i386 code.

> That implementation has a "fast path" that uses a direct asm sequence
> using cmpxchgl, and a "slow path" that invokes spin_lock() and
> spin_unlock().  Lockmeter, of course, will only instrument this "slow
> path".  You simply won't see any 1.4.9 statistics for the "fast path".
> 
> What I did in 1.4.10 was to force everything through the "slow path",
> which means that all those formerly invisible "fast path" spinlock
> acquisitions are now showing up as metered spin_lock/spin_unlock pairs.
>
> So I would argue that 1.4.9 not only aggregates lots of spinlock/unlock
> activity into appearing as atomic_dec_and_lock() calls -- which 1.4.10
> fixes to now make that activity visible in the actual spinlock
> acquisition routine, such as dput() -- but 1.4.9 also completely misses
> quite a few spinlock acquisitions that were going through that "fast
> path".  So I would argue that 1.4.10 actually produces correct
> statistics, not simply *different* statistics, vs. 1.4.9.

The fast path does _not_ take global lock..and I don't think it is a
problem if lockmeter does not measure stats for the fast path. I think it
is more important to get correct stats for global lock acquisitions at-least for
scalability goals. By saying "correct" I mean "as it happens without lockmeter"
Without lockmeter all calls to atomic_dec_and_lock() do not acquire global lock
and lockmeter (v1.4.10) changes this behavior by forcing all calls to 
atomic_dec_and_lock() to slow path. I got mislead due to this as I had to 
investigate why there was 10 fold increase in dcache_lock() acquisitions from
dput().

The problem of aggregation can be solved if the whole of atomic_dec_and_lock()
is made in-line, not only the slow path code. 

Regards,
Maneesh

-- 
Maneesh Soni
IBM Linux Technology Center, 
IBM India Software Lab, Bangalore.
Phone: +91-80-5044999 email: ma...@in...
http://lse.sourceforge.net/locking/rcupdate.html