Re: Another proposal for DAX fault locking

[Mel Gorman]

On Tue, Feb 09, 2016 at 07:46:05PM +0100, Cedric Blancher wrote:
> On 9 February 2016 at 18:24, Jan Kara <jack@xxxxxxx> wrote:
> > Hello,
> >
> > I was thinking about current issues with DAX fault locking [1] (data
> > corruption due to racing faults allocating blocks) and also races which
> > currently don't allow us to clear dirty tags in the radix tree due to races
> > between faults and cache flushing [2]. Both of these exist because we don't
> > have an equivalent of page lock available for DAX. While we have a
> > reasonable solution available for problem [1], so far I'm not aware of a
> > decent solution for [2]. After briefly discussing the issue with Mel he had
> > a bright idea that we could used hashed locks to deal with [2] (and I think
> > we can solve [1] with them as well). So my proposal looks as follows:
> >
> > DAX will have an array of mutexes
> 
> One folly here: Arrays of mutexes NEVER work unless you manage to
> align them to occupy one complete L2/L3 cache line each. Otherwise the
> CPUS will fight over cache lines each time they touch (read or write)
> a mutex, and it then becomes a O^n-like scalability problem if
> multiple mutexes occupy one cache line. It becomes WORSE as more
> mutexes fit into a single cache line and even more worse with the
> number of CPUS accessing such contested lines.
> 

That is a *potential* performance concern although I agree with you in that
mutex's false sharing a cache line would be a problem. However, it is a
performance concern that potentially is alleviated by alternative hashing
where as AFAIK the issues being faced currently are data corruption and
functional issues. I'd take a performance issue over a data corruption
issue any day of the week.

-- 
Mel Gorman
SUSE Labs