linux-kernel - Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread

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Message-ID: <Pine.LNX.4.44L0.1607271450240.1664-100000@iolanthe.rowland.org>
Date:	Wed, 27 Jul 2016 14:54:51 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Oliver Neukum <oneukum@...e.com>,
	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	Geliang Tang <geliangtang@....com>,
	"GeyslanG.Bem@...yakshetra" <geyslan@...il.com>,
	Masanari Iida <standby24x7@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Michal Hocko <MHocko@...e.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Saurabh Karajgaonkar <skarajga@...teon.com>,
	<linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue

On Wed, 27 Jul 2016, Tejun Heo wrote:

> Hello, Oliver.
> 
> On Wed, Jul 27, 2016 at 11:29:56AM +0200, Oliver Neukum wrote:
> > On Wed, 2016-07-27 at 14:50 +0530, Bhaktipriya Shridhar wrote:
> > > The workqueue "workqueue" has multiple workitems which may require
> > > ordering. Hence, a dedicated ordered workqueue has been used.
> > > Since the workqueue is not being used on a memory reclaim path,
> > > WQ_MEM_RECLAIM has not been set.
> > 
> > That is incorrect. The work queue is used by the HCD to handle
> > TDs, which are parts of basic IO. The HCD in turn is used by
> > usb-storage and uas, which are block drivers and those are obviously
> > used on the memory reclaim path.
> 
> Hmm... I didn't know the whole USB stack could operate without
> allocating memory.  Does usb stack have mempools and stuff all the way
> through?

No -- the USB stack does need to allocate memory in order to operate.  
But it is careful to use GFP_NOIO or GFP_ATOMIC for allocations that
might be on the block-device path.

Alan Stern