linux-kernel - Re: [PATCH 2/2] block: Avoid deadlocks with bio allocation by stacking drivers

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Date:	Tue, 11 Sep 2012 12:31:24 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Muthu Kumar <muthu.lkml@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com, axboe@...nel.dk,
	Vivek Goyal <vgoyal@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>, bharrosh@...asas.com,
	david@...morbit.com
Subject: Re: [PATCH 2/2] block: Avoid deadlocks with bio allocation by
 stacking drivers

On Tue, Sep 11, 2012 at 11:58:05AM -0700, Muthu Kumar wrote:
> On Tue, Sep 11, 2012 at 11:45 AM, Tejun Heo <tj@...nel.org> wrote:
> > Hello,
> >
> > On Tue, Sep 11, 2012 at 11:36:28AM -0700, Muthu Kumar wrote:
> >> Does this preserve the CPU from which the bio was submitted
> >> originally. Not familiar with cmwq, may be Tejun can clarify.
> >>
> >> Tejun - the question is, do we honor the rq_affinity with the above
> >> rescue worker implementation?
> >
> > The work item would run from the same CPU but there isn't any
> > mechanism to keep track of the issuing CPU if there are multiple bios
> > to be rescued.  Isn't rq_affinity an optimization hint?  If so, I
> > don't think it matters here.
> >
> 
> Thanks... Just worried about performance impact.
> 
> Kent - Anything to validate that the performance is not impacted would
> be really good. Otherwise, the patch looks great.

Well - there'll only be any performance impact at all when we're memory
constrained enough that GFP_NOWAIT allocations fail, which for these
size allocations definitely isn't normal.

I did test it with forcing everything to use the rescuer, and I also
benchmarked Vivek's version - in any sane configuration, the impact of
punting everything to workqueue is not very noticable (the AHCI
interrupt handler uses more cpu).

> 
> Feel free to add:
> 
> Reviewed-by: Muthukumar Ratty <muthur@...il.com>

Thanks!
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