linux-kernel - Re: [PATCH 1/2] net: Assert proper context while calling napi

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Message-ID: <20250217-weightless-calm-degu-539e59@leitao>
Date: Mon, 17 Feb 2025 08:46:21 -0800
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Hayes Wang <hayeswang@...ltek.com>,
	linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: Assert proper context while calling
 napi_schedule()

On Fri, Feb 14, 2025 at 02:10:11PM -0800, Jakub Kicinski wrote:
> On Fri, 14 Feb 2025 08:43:28 -0800 Breno Leitao wrote:
> > diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
> > index 42f247cbdceec..cd56904a39049 100644
> > --- a/drivers/net/netdevsim/netdev.c
> > +++ b/drivers/net/netdevsim/netdev.c
> > @@ -87,7 +87,7 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >  	if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))
> >  		goto out_drop_cnt;
> >  
> > -	napi_schedule(&rq->napi);
> > +	hrtimer_start(&rq->napi_timer, ns_to_ktime(5), HRTIMER_MODE_REL);
> 
> ns -> us
> 
> We want to leave the timer be in case it's already scheduled.
> Otherwise we'll keep postponing forever under load.
> Double check that hrtime_start() does not reset the time if already
> pending. Maybe hrtimer_start_range_ns(..., 0, us_to_ktime(5), ...)
> would work?

Reading the code, I got the impression that
hrtimer_start_range_ns(..., 0, us_to_ktime(5), ...) will reprogram the
timer anyway.

	hrtimer_start_range_ns() {
		__hrtimer_start_range_ns() {
			remove_hrtimer(timer, base, true, force_local);
			enqueue_hrtimer(timer, new_base, mode);
			...
		}	
	}
	
I think a better solution is to do something as:

	if (!hrtimer_active(&rq->napi_timer))
		hrtimer_start(&rq->napi_timer, us_to_ktime(5), HRTIMER_MODE_REL);