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Thu, 19 Feb 2026 02:36:49 -0800 (PST) Message-ID: Date: Thu, 19 Feb 2026 10:36:48 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] perf bench: Fix initialization of union To: Namhyung Kim , Arnaldo Carvalho de Melo , Ian Rogers , Leo Yan Cc: Jiri Olsa , Adrian Hunter , Peter Zijlstra , Ingo Molnar , LKML , linux-perf-users@vger.kernel.org References: <20260219004417.188434-1-namhyung@kernel.org> Content-Language: en-US From: James Clark In-Reply-To: <20260219004417.188434-1-namhyung@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 19/02/2026 12:44 am, Namhyung Kim wrote: > Recent compilers don't initialize all members (or the largest member) in > an union. Instead it seems to set the first member only. So some perf > bench output shows invalid numbers like below on my system with GCC 15. I couldn't find V1 of this so I wasn't sure if it was discussed already, but wasn't the plan to fix this with a compiler option: https://lore.kernel.org/linux-perf-users/20250319110454.3230687-1-leo.yan@arm.com/ and https://lore.kernel.org/linux-perf-users/20250320105235.3498106-1-leo.yan@arm.com/ > > Before: > $ perf bench mem mmap > # Running 'mem/mmap' benchmark: > # function 'demand' (Demand loaded mmap()) > # Copying 1MB bytes ... > > 0.011127 bytes/sec > # function 'populate' (Eagerly populated mmap()) > # Copying 1MB bytes ... > > 0.011127 bytes/sec > > After: > $ perf bench mem mmap > # Running 'mem/mmap' benchmark: > # function 'demand' (Demand loaded mmap()) > # Copying 1MB bytes ... > > 2.209417 GB/sec > # function 'populate' (Eagerly populated mmap()) > # Copying 1MB bytes ... > > 7.875504 GB/sec > > Because the first member of bench_clock is u64 ('cycles'), the default > struct timeval wasn't fully initialized. Let's use memset to reset the > whole memory. > > Signed-off-by: Namhyung Kim > --- > tools/perf/bench/mem-functions.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/tools/perf/bench/mem-functions.c b/tools/perf/bench/mem-functions.c > index 2908a3a796c932d0..676c8d18f4c2e259 100644 > --- a/tools/perf/bench/mem-functions.c > +++ b/tools/perf/bench/mem-functions.c > @@ -193,9 +193,10 @@ static void __bench_mem_function(struct bench_mem_info *info, struct bench_param > { > const struct function *r = &info->functions[r_idx]; > double result_bps = 0.0; > - union bench_clock rt = { 0 }; > + union bench_clock rt; > void *src = NULL, *dst = NULL; > > + memset(&rt, 0, sizeof(rt)); > printf("# function '%s' (%s)\n", r->name, r->desc); > > if (r->fn.init && r->fn.init(info, p, &src, &dst))