netdev - Re: [PATCH net] net: usbnet: fix softirq storm on suspend

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Date:	Tue, 4 Sep 2012 12:19:28 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Oliver Neukum <oneukum@...e.de>
Subject: Re: [PATCH net] net: usbnet: fix softirq storm on suspend

On Mon, Sep 3, 2012 at 4:26 PM, Bjørn Mork <bjorn@...k.no> wrote:
> Suspending an open usbnet device results in constant
> rescheduling of usbnet_bh.
>
> commit 65841fd5 "usbnet: handle remote wakeup asap"
> refactored the usbnet_bh code to allow sharing the
> urb allocate and submit code with usbnet_resume. In
> this process, a test for, and immediate return on,
> ENOLINK from rx_submit was unintentionally dropped.
>
> The rx queue will not grow if rx_submit fails,
> making usbnet_bh reschedule itself.  This results
> in a softirq storm if the error is persistent.
> rx_submit translates the usb_submit_urb error
> EHOSTUNREACH into ENOLINK, so this is an expected
> and persistent error for a suspended device. The
> old code tested for this condition and avoided
> rescheduling.  Putting this test back.
>
> Cc: <stable@...r.kernel.org> # v3.5
> Cc: Ming Lei <ming.lei@...onical.com>
> Cc: Oliver Neukum <oneukum@...e.de>
> Signed-off-by: Bjørn Mork <bjorn@...k.no>
> ---
> Sorry for not noticing this before, but commit 65841fd5
> makes usbnet autosuspend completely unusable.  The device
> is suspended fine, but burning one CPU core at full load
> uses a tiny bit more power making the power saving
> negative...

I am wondering how you can reproduce the issue.

usbnet_terminate_urbs is called inside usbnet_suspend to
consume all URBs and SKBs, and rx_alloc_submit can't be
called during the period because of !netif_device_present().
Once usbnet_terminate_urbs and netif_device_attach() are
completed, who will schedule tasklet to trigger rx_alloc_submit?

>
> I hope this can go into 3.6 and 3.5-stable ASAP. It is
> a hard to notice regression, but all the same a serious
> one.

Thanks,
--
Ming Lei
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