Re: [RFC][PATCH 1/3] sched, block: Move unplug

From: Jens Axboe
Date: Wed Jun 22 2011 - 12:04:49 EST

On 2011-06-22 17:08, Vivek Goyal wrote:
> On Wed, Jun 22, 2011 at 04:38:01PM +0200, Peter Zijlstra wrote:
>> On Wed, 2011-06-22 at 16:30 +0200, Thomas Gleixner wrote:
>>>> It is supposedly a real problem, not just an inkling. It's not about
>>>> recursing indefinitely, the plug is fairly bounded. But the IO dispatch
>>>> path can be pretty deep, and if you hit that deep inside the reclaim or
>>>> file system write path, then you get dangerously close. Dave Chinner
>>>> posted some numbers in the 2.6.39-rc1 time frame showing how close we
>>>> got.
>>> Fair enough.
>>> Ok. So we move it out and keep the from_scheduler flag so that code
>>> does not go down the IO path from there.
>> Won't punting the plug to a worker thread wreck all kinds of io
>> accounting due to the wrong task doing the actual io submission?
> I think all the accounting will the done in IO submission path and
> while IO is added to plug. This is just plug flush so should not
> have effect on accounting.

Exactly, this is just the insert operation, so no worries there. The
request are fully "formulated".

Jens Axboe

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