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([2a0c:5a80:1b14:b500:abb:f9d1:7bc2:3db8]) by smtp.gmail.com with ESMTPSA id f11-20020a7bcc0b000000b0037e0c362b6dsm5052933wmh.31.2022.03.04.06.36.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Mar 2022 06:36:59 -0800 (PST) Message-ID: Subject: Re: [PATCH] tracing/osnoise: Force quiescent states while tracing From: Nicolas Saenz Julienne To: Daniel Bristot de Oliveira 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> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.42.4 (3.42.4-1.fc35) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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