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Tue, 03 Mar 2026 13:32:24 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Tue, 3 Mar 2026 22:32:23 +0100 To: sun jian Cc: Jiri Olsa , Andrii Nakryiko , Shuah Khan , Eduard Zingerman , Alexei Starovoitov , Daniel Borkmann , bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/2] selftests/bpf: bpf_cookie: make perf_event subtest trigger reliably Message-ID: References: <20260228074555.122950-1-sun.jian.kdev@gmail.com> <20260228074555.122950-3-sun.jian.kdev@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Mar 03, 2026 at 10:15:26AM +0800, sun jian wrote: > On Mon, Mar 2, 2026 at 6:02 PM Jiri Olsa wrote: > > > > On Sat, Feb 28, 2026 at 03:45:55PM +0800, Sun Jian wrote: > > > The perf_event subtest relies on SW_CPU_CLOCK sampling to trigger the BPF > > > -static void burn_cpu(void) > > > +static void burn_cpu(long loops) > > > > nit, there's another burn_cpu in prog_tests/perf_link.c, > > we could add it to trace_helpers.c or test_progs.c > > > > happy to refactor into a shared helper if maintainers prefer, but I keep it > local to minimize the diff. > > > > { > > > - volatile int j = 0; > > > + long j = 0; > > > cpu_set_t cpu_set; > > > - int i, err; > > > + long i; > > > + int err; > > > > > > /* generate some branches on cpu 0 */ > > > CPU_ZERO(&cpu_set); > > > @@ -443,9 +445,10 @@ static void burn_cpu(void) > > > err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set); > > > ASSERT_OK(err, "set_thread_affinity"); > > > > > > - /* spin the loop for a while (random high number) */ > > > - for (i = 0; i < 1000000; ++i) > > > + for (i = 0; i < loops; ++i) { > > > ++j; > > > + barrier(); > > > > what's the rationale for barrier call in here, > > together with the volatile change above? > > > > The burn_cpu() loop is only meant to consume CPU time to reliably trigger the > SW_CPU_CLOCK perf_event overflow. With an side-effect-free loop, the > compiler may optimize the loop away or significantly shrink it under -O2. > > The old version relied on volatile to prevent the loop from being optimized, but > checkpatch warns against it. Using barrier() achieves the same goal — keep the > loop intact as a CPU-burn — while making the intent more explicit. ok, would be great to have this in the changelog, other than that: Acked-by: Jiri Olsa thanks, jirka