Re: [patch 6/8] raid5: make_request use batch stripe release

[Shaohua Li <shli@kernel.org>]

On Thu, Jun 07, 2012 at 11:23:45AM +1000, NeilBrown wrote:
> On Mon, 04 Jun 2012 16:01:58 +0800 Shaohua Li <shli@kernel.org> wrote:
> 
> > make_request() does stripe release for every stripe and the stripe usually has
> > count 1, which makes previous release_stripe() optimization not work. In my
> > test, this release_stripe() becomes the heaviest pleace to take
> > conf->device_lock after previous patches applied.
> > 
> > Below patch makes stripe release batch. When maxium strips of a batch reach,
> > the batch will be flushed out. Another way to do the flush is when unplug is
> > called.
> > 
> > Signed-off-by: Shaohua Li <shli@fusionio.com>
> 
> I like the idea of a batched release.
> I don't like the per-cpu variables... and I don't think it is safe to only
> allocate them for_each_present_cpu without support cpu-hot-plug.
> 
> I would much rather keep a list of stripes (linked on ->lru) in struct
> md_plug_cb (or maybe in some structure which contains that) and release them
> all on unplug - and only on unplug.
> 
> Maybe pass a size to mddev_check_unplugged, and it allocates that much more
> space.  Get mddev_check_unplugged to return the md_plug_cb structure.
> If the new space is NULL, then list_head_init it, and change the cb.callback
> to a raid5 specific function.
> Then add any stripe to the md_plug_cb, and in the unplug function, release
> them all.
> 
> Does that make sense?
> 
> Also I would rather the batched stripe release code were defined in the same
> patch that used it.  It isn't big enough to justify a separate patch.

The stripe->lru need protection of device_lock, so I can't use a list. An array
is preferred. I really didn't like the idea to allocate memory especially when
allocating an array. I'll fix the code for cpuhotplug.