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Message-ID: <66a81295-ab6f-41f4-a3da-8b5003634c6a@paulmck-laptop>
Date: Tue, 27 Feb 2024 20:42:24 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>, Yan Zhai <yan@...udflare.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 Tue, Feb 27, 2024 at 07:10:01PM -0800, Jakub Kicinski wrote:
> On Tue, 27 Feb 2024 10:32:22 -0800 Paul E. McKenney wrote:
> > > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > > > + rcu_softirq_qs();
> > > > +
> > > > local_bh_enable();
> > > >
> > > > if (!repoll)
> > >
> > > Hmm....
> > > Why napi_busy_loop() does not have a similar problem ?
> > >
> > > It is unclear why rcu_all_qs() in __cond_resched() is guarded by
> > >
> > > #ifndef CONFIG_PREEMPT_RCU
> > > rcu_all_qs();
> > > #endif
> >
> > The theory is that PREEMPT_RCU kernels have preemption, and get their
> > quiescent states that way.
>
> But that doesn't work well enough?
>
> Assuming that's the case why don't we add it with the inverse ifdef
> condition next to the cond_resched() which follows a few lines down?
>
> skb_defer_free_flush(sd);
> +
> + if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> + rcu_softirq_qs();
> +
> local_bh_enable();
>
> if (!repoll)
> break;
>
> cond_resched();
> }
>
> We won't repoll majority of the time.
I am not completely clear on what you are proposing, but one complication
is that We need preemption disabled across calls to rcu_softirq_qs()
and we cannot have preemption disabled across calls to cond_resched().
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?
All in all, I suspect that I am missing your point.
Thanx, Paul
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