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Wed, 26 Aug 2020 14:37:26 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 88462B2064; Wed, 26 Aug 2020 14:37:22 +0000 (GMT) Received: from localhost.localdomain (unknown [9.199.63.208]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 26 Aug 2020 14:37:22 +0000 (GMT) Subject: Re: [RFC] perf/core: Fixes hung issue on perf stat command during cpu hotplug To: Arnaldo Carvalho de Melo , Peter Zijlstra 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, brho@google.com, srikar@linux.vnet.ibm.com References: <20200826093236.446024-1-kjain@linux.ibm.com> <20200826132107.GH1059382@kernel.org> From: kajoljain Message-ID: <135a1238-9a43-8335-e8a6-961678e95f65@linux.ibm.com> Date: Wed, 26 Aug 2020 20:07:20 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200826132107.GH1059382@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-08-26_09:2020-08-26,2020-08-26 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 spamscore=0 priorityscore=1501 mlxlogscore=999 bulkscore=0 impostorscore=0 phishscore=0 malwarescore=0 mlxscore=0 clxscore=1015 suspectscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2008260108 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/26/20 6:51 PM, Arnaldo Carvalho de Melo wrote: > Em Wed, Aug 26, 2020 at 03:02:36PM +0530, Kajol Jain escreveu: >> 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 ) > > Peter, this is kernel stuff, can you take a look? > > - Arnaldo > >> 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 | 6 +++--- >> 1 file changed, 3 insertions(+), 3 deletions(-) >> >> diff --git a/kernel/events/core.c b/kernel/events/core.c >> index 5bfe8e3c6e44..330c53f7df9c 100644 >> --- a/kernel/events/core.c >> +++ b/kernel/events/core.c >> @@ -115,9 +115,9 @@ 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 != -EAGAIN) >> + if (!ret) >> + ret = data.ret; >> + else if (ret != -EAGAIN) >> break; >> >> cond_resched(); >> -- Hi, Sorry for the confusion, I send wrong version of the patch. We don't have else in second condition. The right patch changes are: diff --git a/kernel/events/core.c b/kernel/events/core.c index 5bfe8e3c6e44..53d960394af9 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) + ret = data.ret; if (ret != -EAGAIN) break; I will again send the patch, please ignore this one. Thanks, Kajol Jain >> 2.26.2 >> >