linux-kernel - Re: [PATCH] USB: HCD: Fix URB giveback issue in tasklet function

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Message-ID: <YtlZn74MDEj+1On9@rowland.harvard.edu>
Date:   Thu, 21 Jul 2022 09:50:23 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Weitao Wang <WeitaoWang-oc@...oxin.com>
Cc:     gregkh@...uxfoundation.org, kishon@...com, dianders@...omium.org,
        s.shtylyov@....ru, mka@...omium.org, ming.lei@...onical.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        tonywwang@...oxin.com, weitaowang@...oxin.com,
        CobeChen@...oxin.com, TimGuo@...oxin.com
Subject: Re: [PATCH] USB: HCD: Fix URB giveback issue in tasklet function

On Thu, Jul 21, 2022 at 02:08:33PM +0800, Weitao Wang wrote:
> Usb core introduce the mechanism of giveback of URB in tasklet context to
> reduce hardware interrupt handling time. On some test situation(such as
> FIO with 4KB block size), when tasklet callback function called to
> giveback URB, interrupt handler add URB node to the bh->head list also.
> If check bh->head list again after finish all URB giveback of local_list,
> then it may introduce a "dynamic balance" between giveback URB and add URB
> to bh->head list. This tasklet callback function may not exit for a long
> time, which will cause other tasklet function calls to be delayed. Some
> real-time applications(such as KB and Mouse) will see noticeable lag.
> 
> Fix this issue by taking new URBs giveback in next tasklet function call.

The patch also replaces the local high_prio_bh variable with a new
bh->high_prio structure member.  This should be mentioned in the patch 
description.

Alan Stern