netdev - RE: [PATCH net-next] net: link_watch: prevent starvation when processing linkwatch wq

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Date:   Wed, 29 May 2019 08:12:35 +0000
From:   Salil Mehta <salil.mehta@...wei.com>
To:     linyunsheng <linyunsheng@...wei.com>,
        Stephen Hemminger <stephen@...workplumber.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: RE: [PATCH net-next] net: link_watch: prevent starvation when
 processing linkwatch wq

> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On Behalf Of Yunsheng Lin
> Sent: Tuesday, May 28, 2019 2:04 AM
> 
> On 2019/5/27 22:58, Stephen Hemminger wrote:
> > On Mon, 27 May 2019 09:47:54 +0800
> > Yunsheng Lin <linyunsheng@...wei.com> wrote:
> >
> >> When user has configured a large number of virtual netdev, such
> >> as 4K vlans, the carrier on/off operation of the real netdev
> >> will also cause it's virtual netdev's link state to be processed
> >> in linkwatch. Currently, the processing is done in a work queue,
> >> which may cause worker starvation problem for other work queue.


I think we had already discussed about this internally and using separate
workqueue with WQ_UNBOUND should solve this problem. HNS3 driver was sharing
workqueue with the system workqueue. 


> >> This patch releases the cpu when link watch worker has processed
> >> a fixed number of netdev' link watch event, and schedule the
> >> work queue again when there is still link watch event remaining.


We need proper examples/use-cases because of which we require above
kind of co-operative scheduling. Touching the common shared queue logic
which solid argument might invite for more problem to other modules.


> >> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> >
> > Why not put link watch in its own workqueue so it is scheduled
> > separately from the system workqueue?
> 
> From testing and debuging, the workqueue runs on the cpu where the
> workqueue is schedule when using normal workqueue, even using its
> own workqueue instead of system workqueue. So if the cpu is busy
> processing the linkwatch event, it is not able to process other
> workqueue' work when the workqueue is scheduled on the same cpu.
> 
> Using unbound workqueue may solve the cpu starvation problem.

[...]

> But the __linkwatch_run_queue is called with rtnl_lock, so if it
> takes a lot time to process, other need to take the rtnl_lock may
> not be able to move forward.

Please help me in understanding, Are you trying to pitch this patch
to solve more general system issue OR still your argument/concern
is related to the HNS3 driver problem mentioned in this patch?

Salil.