netdev - Re: [PATCH v5 net 1/3] rcu: add a helper to report consolidated flavor QS

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Date: Fri, 22 Mar 2024 12:24:13 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Yan Zhai <yan@...udflare.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, 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>,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...udflare.com,
	Joel Fernandes <joel@...lfernandes.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, mark.rutland@....com,
	Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH v5 net 1/3] rcu: add a helper to report consolidated
 flavor QS

On 2024-03-19 13:44:34 [-0700], Yan Zhai wrote:
> index 16f519914415..17d7ed5f3ae6 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -247,6 +247,37 @@ do { \
>  	cond_resched(); \
>  } while (0)
>  
> +/**
> + * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
> + * @old_ts: jiffies at start of processing.
> + *
> + * This helper is for long-running softirq handlers, such as NAPI threads in
> + * networking. The caller should initialize the variable passed in as @old_ts
> + * at the beginning of the softirq handler. When invoked frequently, this macro
> + * will invoke rcu_softirq_qs() every 100 milliseconds thereafter, which will
> + * provide both RCU and RCU-Tasks quiescent states. Note that this macro
> + * modifies its old_ts argument.
> + *
> + * Because regions of code that have disabled softirq act as RCU read-side
> + * critical sections, this macro should be invoked with softirq (and
> + * preemption) enabled.
> + *
> + * The macro is not needed when CONFIG_PREEMPT_RT is defined. RT kernels would
> + * have more chance to invoke schedule() calls and provide necessary quiescent
> + * states. As a contrast, calling cond_resched() only won't achieve the same
> + * effect because cond_resched() does not provide RCU-Tasks quiescent states.
> + */

Paul, so CONFIG_PREEMPTION is affected but CONFIG_PREEMPT_RT is not.
Why does RT have more scheduling points?
The RCU-Tasks thread is starving and yet there is no wake-up, correct?
Shouldn't cond_resched() take care of RCU-Tasks's needs, too?
This function is used by napi_threaded_poll() which is not invoked in
softirq it is a simple thread which does disable BH but this is it. Any
pending softirqs are served before the cond_resched().

This napi_threaded_poll() case _basically_ a busy thread doing a lot of
work and delaying RCU-Tasks as far as I understand. The same may happen
to other busy-worker which have cond_resched() between works, such as
the kworker. Therefore I would expect to have some kind of timeout at
which point NEED_RESCHED is set so that cond_resched() can do its work.

> +#define rcu_softirq_qs_periodic(old_ts) \
> +do { \
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> +	    time_after(jiffies, (old_ts) + HZ / 10)) { \
> +		preempt_disable(); \
> +		rcu_softirq_qs(); \
> +		preempt_enable(); \
> +		(old_ts) = jiffies; \
> +	} \
> +} while (0)
> +
>  /*
>   * Infrastructure to implement the synchronize_() primitives in
>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.

Sebastian