Re: v4.9, 4.4-final: 28 bioset threads on small notebook, 36 threads on cellphone

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Feb 06 2017 at  9:49pm -0500,
Kent Overstreet <kent.overstreet@gmail.com> wrote:

> On Mon, Feb 06, 2017 at 04:47:24PM -0900, Kent Overstreet wrote:
> > On Mon, Feb 06, 2017 at 01:53:09PM +0100, Pavel Machek wrote:
> > > Still there on v4.9, 36 threads on nokia n900 cellphone.
> > > 
> > > So.. what needs to be done there?
> 
> > But, I just got an idea for how to handle this that might be halfway sane, maybe
> > I'll try and come up with a patch...
> 
> Ok, here's such a patch, only lightly tested:
> 
> -- >8 --
> Subject: [PATCH] block: Make rescuer threads per request_queue, not per bioset
> 
> Note: this patch is very lightly tested.
> 
> Also, trigger rescuing whenever with bios on current->bio_list, instead
> of only when we block in bio_alloc_bioset(). This is more correct, and
> should result in fewer rescuer threads.
> 
> XXX: The current->bio_list plugging needs to be unified with the
> blk_plug mechanism.
> 
> TODO: If we change normal request_queue drivers to handle arbitrary size
> bios by processing requests incrementally, instead of splitting bios,
> then we can get rid of rescuer threads from those devices.

Hi Kent,

I really appreciate you working on this further.  Thanks.

As I think you're probably already aware, a long standing issue with the
per bio_set rescuer is this bug (which manifests in dm-snapshot
deadlocks): https://bugzilla.kernel.org/show_bug.cgi?id=119841

Please also see this patch header, from a private branch from a while
ago, that describes the problem in detail:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=cd2c760b5a609e2aaf3735a7b9503a953535c368

Would welcome your consideration of that BZ as you think further and/or
iterate on this line of work.

Mike