linux-kernel - Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers

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Date:	Tue, 4 Sep 2012 15:26:19 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
cc:	Vivek Goyal <vgoyal@...hat.com>, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com, tj@...nel.org,
	bharrosh@...asas.com, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by
 stacking drivers

On Mon, 3 Sep 2012, Kent Overstreet wrote:

> On Mon, Sep 03, 2012 at 04:41:37PM -0400, Mikulas Patocka wrote:
> > ... or another possibility - start a timer when something is put to 
> > current->bio_list and use that timer to pop entries off current->bio_list 
> > and submit them to a workqueue. The timer can be cpu-local so only 
> > interrupt masking is required to synchronize against the timer.
> > 
> > This would normally run just like the current kernel and in case of 
> > deadlock, the timer would kick in and resolve the deadlock.
> 
> Ugh. That's a _terrible_ idea.
> 
> Remember the old plugging code? You ever have to debug performance
> issues caused by it?

Yes, I do remember it (and I fixed one bug that resulted in missed unplug 
and degraded performance).

But currently, deadlocks due to exhausted mempools and bios being stalled 
in current->bio_list don't happen (or do happen below so rarely that they 
aren't reported).

If we add a timer, it will turn a deadlock into an i/o delay, but it can't 
make things any worse.

BTW. can these new-style timerless plugs introduce deadlocks too? What 
happens when some bios are indefinitely delayed because their requests are 
held in a plug and a mempool runs out?

> > > I could be convinced, but right now I prefer my solution.
> > 
> > It fixes bio allocation problem, but not other similar mempool problems in 
> > dm and md.
> 
> I looked a bit more, and actually I think the rest of the problem is
> pretty limited in scope - most of those mempool allocations are per
> request, not per split.
> 
> I'm willing to put some time into converting dm/md over to bioset's
> front_pad. I'm having to learn the code for the immutable biovec work,
> anyways.

Currently, dm targets allocate request-specific data from target-specific 
mempool. mempools are in dm-crypt, dm-delay, dm-mirror, dm-snapshot, 
dm-thin, dm-verity. You can change it to allocate request-specific data 
with the bio.

Mikulas
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