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

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Message-ID: <CAFR8uedk3KpqhJuMn5rVuDbbWz=VOqt=O7ssFqugHTcw=EZsFw@mail.gmail.com>
Date:	Tue, 11 Sep 2012 13:00:31 -0700
From:	Muthu Kumar <muthu.lkml@...il.com>
To:	Kent Overstreet <koverstreet@...gle.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

Kent,

On Tue, Sep 11, 2012 at 12:31 PM, Kent Overstreet
<koverstreet@...gle.com> wrote:
> 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.


Agreed. If we are in this situation, we are already running in
degraded mode. So performance is not in question.

Regards,
Muthu
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