Re: [PATCH 2/7] dm: add reserved_rq_based_ios module parameter

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Sep 12 2013 at  7:27pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 12 Sep 2013, Mike Snitzer wrote:
> 
> > On Thu, Sep 12 2013 at  6:45pm -0400,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > I think this is too complicated.
> > > 
> > > dm-bufio uses similar approach like this patch - but in dm-bufio the code 
> > > path is performance critical, we don't want to recalculate memory size 
> > > with each i/o request.
> > > 
> > > Here it is not performance critical, so I'd drop the mutex, drop the 
> > > latch, drop the function __reserved_request_based_ios_refresh and add only 
> > > these lines:
> > > 
> > > pool_size = ACCESS_ONCE(reserved_rq_based_ios);
> > > if (!pool_size) pool_size = RESERVED_REQUEST_BASED_IOS;
> > 
> > Could be I'm missing something but isn't the locking about correctness
> > rather than performance?
> 
> If the user changes the value while you are allocating the mempool, you 
> can allocate the mempool with the old or new value, it doesn't matter. 
> There is nothing to lock against.
> 
> > Also, using the latch enables the dm_get_reserved_rq_based_ios()
> > interface that is used in patch 5.
> > 
> > Mike
> 
> You can just define dm_get_reserved_rq_based_ios as this:
> unsigned dm_get_reserved_rq_based_ios(void)
> {
> 	unsigned pool_size = ACCESS_ONCE(reserved_rq_based_ios);
> 	if (!pool_size) pool_size = RESERVED_REQUEST_BASED_IOS;
> 	return pool_size;
> }
> ... and call it when you need the pool size.
> 
> The purpose of the latch in dm-bufio is to avoid memory size recalculation 
> with each buffer allocation. Here, there is nothing to recalculate and the 
> code path is not performance-critical.

OK, I'll revise the patch.

Thanks,
Mike