RE: [PATCH v2 net-next] net: link_watch: prevent starvation when processing linkwatch wq

From: Salil Mehta
Date: Fri May 31 2019 - 05:58:05 EST


> From: netdev-owner@xxxxxxxxxxxxxxx On Behalf Of Yunsheng Lin
> Sent: Friday, May 31, 2019 10:01 AM
> To: davem@xxxxxxxxxxxxx
> Cc: hkallweit1@xxxxxxxxx; f.fainelli@xxxxxxxxx;
> stephen@xxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Linuxarm <linuxarm@xxxxxxxxxx>
> Subject: [PATCH v2 net-next] net: link_watch: prevent starvation when
> processing linkwatch wq
>
> 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 cpu and rtnl locking starvation problem.
>
> This patch releases the cpu and rtnl lock when link watch worker
> has processed a fixed number of netdev' link watch event.
>
> Currently __linkwatch_run_queue is called with rtnl lock, so
> enfore it with ASSERT_RTNL();


Typo enfore --> enforce ?



> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> ---
> V2: use cond_resched and rtnl_unlock after processing a fixed
> number of events
> ---
> net/core/link_watch.c | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/net/core/link_watch.c b/net/core/link_watch.c
> index 7f51efb..07eebfb 100644
> --- a/net/core/link_watch.c
> +++ b/net/core/link_watch.c
> @@ -168,9 +168,18 @@ static void linkwatch_do_dev(struct net_device
> *dev)
>
> static void __linkwatch_run_queue(int urgent_only)
> {
> +#define MAX_DO_DEV_PER_LOOP 100
> +
> + int do_dev = MAX_DO_DEV_PER_LOOP;
> struct net_device *dev;
> LIST_HEAD(wrk);
>
> + ASSERT_RTNL();
> +
> + /* Give urgent case more budget */
> + if (urgent_only)
> + do_dev += MAX_DO_DEV_PER_LOOP;
> +
> /*
> * Limit the number of linkwatch events to one
> * per second so that a runaway driver does not
> @@ -200,6 +209,14 @@ static void __linkwatch_run_queue(int urgent_only)
> }
> spin_unlock_irq(&lweventlist_lock);
> linkwatch_do_dev(dev);
> +


A comment like below would be helpful in explaining the reason of the code.

/* This function is called with rtnl_lock held. If excessive events
* are present as part of the watch list, their processing could
* monopolize the rtnl_lock and which could lead to starvation in
* other modules which want to acquire this lock. Hence, co-operative
* scheme like below might be helpful in mitigating the problem.
* This also tries to be fair CPU wise by conditional rescheduling.
*/


> + if (--do_dev < 0) {
> + rtnl_unlock();
> + cond_resched();
> + do_dev = MAX_DO_DEV_PER_LOOP;
> + rtnl_lock();
> + }
> +
> spin_lock_irq(&lweventlist_lock);
> }