linux-kernel - Re: [PATCH] tracing/osnoise: Force quiescent states while tracing

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Date:   Mon, 28 Feb 2022 10:45:23 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     bristot@...nel.org, paulmck@...nel.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, mtosatti@...hat.com
Subject: Re: [PATCH] tracing/osnoise: Force quiescent states while tracing

On Mon, 28 Feb 2022 15:14:23 +0100
Nicolas Saenz Julienne <nsaenzju@...hat.com> wrote:

> @@ -1386,6 +1389,22 @@ static int run_osnoise(void)
>  					osnoise_stop_tracing();
>  		}
>  
> +		/*
> +		 * Check if we're the only ones running on this nohz_full CPU
> +		 * and that we're on a PREEMPT_RCU setup. If so, let's fake a
> +		 * QS since there is no way for RCU to know we're not making
> +		 * use of it.
> +		 *
> +		 * Otherwise it'll be done through cond_resched().
> +		 */
> +		if (IS_ENABLED(CONFIG_PREEMPT_RCU) &&
> +		    !housekeeping_cpu(raw_smp_processor_id(), HK_FLAG_MISC) &&
> +		    tick_nohz_tick_stopped()) {
> +			local_irq_save(flags);
> +			rcu_momentary_dyntick_idle();
> +			local_irq_restore(flags);
> +		}
> +

This looks very specific and a corner case. Something that depends on how
RCU works. This really should be in the RCU code such that if something
changes, RCU maintainers are aware of this and can update this too.

I wonder if this is similar to what we have in trace_benchmark(). Would
using: cond_resched_tasks_rcu_qs() work for you?

-- Steve


>  		/*