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

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Date:   Fri, 4 Mar 2022 15:28:54 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        mtosatti@...hat.com, paulmck@...nel.org, rostedt@...dmis.org
Subject: Re: [PATCH] tracing/osnoise: Force quiescent states while tracing

On 2/28/22 15:14, Nicolas Saenz Julienne wrote:
> At the moment running osnoise on an isolated CPU and a PREEMPT_RCU
> kernel might have the side effect of extending grace periods too much.
> This will eventually entice RCU to schedule a task on the isolated CPU
> to end the overly extended grace period, adding unwarranted noise to the
> CPU being traced in the process.
> 
> So, check if we're the only ones running on this isolated CPU and that
> we're on a PREEMPT_RCU setup. If so, let's force quiescent states in
> between measurements.
> 
> Non-PREEMPT_RCU setups don't need to worry about this as osnoise main
> loop's cond_resched() will go though a quiescent state for them.
> 
> Note that this same exact problem is what extended quiescent states were
> created for. But adapting them to this specific use-case isn't trivial
> as it'll imply reworking entry/exit and dynticks/context tracking code.

Hey Nicolas,

While testing this patch with rtla osnoise on the 5.17.0-rc6-rt10+, when I hit
^c on osnoise top, the system freezes :-/.

Could you try that on your system?

-- Daniel