netdev - Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

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Message-ID: <CAO3-PbrFSrQZQczp7nEj=HLn4GLL+UY0SUCJentgOAJ480dqRg@mail.gmail.com>
Date: Wed, 28 Feb 2024 09:57:13 -0600
From: Yan Zhai <yan@...udflare.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>, 
	Simon Horman <horms@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Coco Li <lixiaoyan@...gle.com>, Wei Wang <weiwan@...gle.com>, 
	Alexander Duyck <alexanderduyck@...com>, Hannes Frederic Sowa <hannes@...essinduktion.org>, 
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org, bpf@...r.kernel.org, 
	kernel-team@...udflare.com
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

On Wed, Feb 28, 2024 at 9:35 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 28 Feb 2024 07:15:42 -0800 Paul E. McKenney wrote:
> > > > Another complication is that although CONFIG_PREEMPT_RT kernels are
> > > > built with CONFIG_PREEMPT_RCU, the reverse is not always the case.
> > > > And if we are not repolling, don't we have a high probability of doing
> > > > a voluntary context when we reach napi_thread_wait() at the beginning
> > > > of that loop?
> > >
> > > Very much so, which is why adding the cost of rcu_softirq_qs()
> > > for every NAPI run feels like an overkill.
> >
> > Would it be better to do the rcu_softirq_qs() only once every 1000 times
> > or some such?  Or once every HZ jiffies?
> >
> > Or is there a better way?
>
> Right, we can do that. Yan Zhai, have you measured the performance
> impact / time spent in the call?
For the case it hits the problem, the __napi_poll itself is usually
consuming much of the cycles, so I didn't notice any difference in
terms of tput. And it is in fact repolling all the time as the
customer traffic might not implement proper backoff. So using a loop
counter or jiffies  to cap the number of invocations sounds like a
decent improvement.

Let me briefly check the overhead in the normal case, too

Yan