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Message-ID: <b5e1f0d14a48c0815863af623cedbdc8c1bc255f.camel@redhat.com>
Subject: Re: [PATCH] tracing/osnoise: Force quiescent states while tracing
From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     Daniel Bristot de Oliveira <bristot@kernel.org>
Cc:     mingo@redhat.com, linux-kernel@vger.kernel.org,
        mtosatti@redhat.com, paulmck@kernel.org, rostedt@goodmis.org
Date:   Fri, 04 Mar 2022 15:36:57 +0100
In-Reply-To: <78d798aa-388c-70bc-4227-985ba76c1d7e@kernel.org>
References: <20220228141423.259691-1-nsaenzju@redhat.com>
         <78d798aa-388c-70bc-4227-985ba76c1d7e@kernel.org>
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On Fri, 2022-03-04 at 15:28 +0100, Daniel Bristot de Oliveira wrote:
> 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?

Yes of course, I'll get a build going.

Were you using nohz_full?

-- 
Nicolás Sáenz