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

[Salil Mehta]

> 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);
>  	}