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[65.175.178.104]) by smtp.gmail.com with ESMTPSA id t1sm1876216qkt.119.2020.08.26.08.14.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Aug 2020 08:14:26 -0700 (PDT) Subject: Re: [RFC v2] perf/core: Fixes hung issue on perf stat command during cpu hotplug To: Kajol Jain , acme@kernel.org, peterz@infradead.org Cc: jolsa@redhat.com, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, maddy@linux.ibm.com, mingo@redhat.com, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, namhyung@kernel.org, daniel@iogearbox.net, srikar@linux.vnet.ibm.com References: <20200826145411.489169-1-kjain@linux.ibm.com> From: Barret Rhoden Message-ID: <19fac9f1-4662-b695-3bf3-60ddd7133594@google.com> Date: Wed, 26 Aug 2020 11:14:25 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200826145411.489169-1-kjain@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi - On 8/26/20 10:54 AM, Kajol Jain wrote: > Commit 2ed6edd33a21 ("perf: Add cond_resched() to task_function_call()") > added assignment of ret value as -EAGAIN in case function > call to 'smp_call_function_single' fails. > For non-zero ret value, it did > 'ret = !ret ? data.ret : -EAGAIN;', which always > assign -EAGAIN to ret and make second if condition useless. > > In scenarios like when executing a perf stat with --per-thread option, and > if any of the monitoring cpu goes offline, the 'smp_call_function_single' > function could return -ENXIO, and with the above check, > task_function_call hung and increases CPU > usage (because of repeated 'smp_call_function_single()') > > Recration scenario: > # perf stat -a --per-thread && (offline a CPU ) > > Patch here removes the tertiary condition added as part of that > commit and added a check for NULL and -EAGAIN. > > Fixes: 2ed6edd33a21("perf: Add cond_resched() to task_function_call()") > Signed-off-by: Kajol Jain > Reported-by: Srikar Dronamraju > --- > kernel/events/core.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > Changelog: > - Remove addition of else in the first patch for > if(ret != -EAGAIN) condition. > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 856d98c36f56..fe104fee097a 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -115,8 +115,8 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info) > for (;;) { > ret = smp_call_function_single(task_cpu(p), remote_function, > &data, 1); > - ret = !ret ? data.ret : -EAGAIN; > - > + if(!ret) ^ Minor nit, you need a space after the if. Also, the comment for the function says it returns @func's return val or -ESRCH. You could also add -ENXIO to that. Thanks for the fix. Reviewed-by: Barret Rhoden > + ret = data.ret; > if (ret != -EAGAIN) > break; > >