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

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Message-ID: <09fed539-909a-f274-1a73-428dc7439d69@suse.com>
Date:   Thu, 21 Jul 2022 10:00:43 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Weitao Wang <WeitaoWang-oc@...oxin.com>, stern@...land.harvard.edu,
        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
Cc:     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 21.07.22 08:08, 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.
> 

Hi,

ow do you know usb_hcd_giveback_urb() will be called in time to process
isoc URBs in time, if you leave them on the list? In fact how do
you be sure it will be called at all? I can see no upper time limit
on that.

	Regards
		Oliver