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Thu, 8 Oct 2020 12:25:41 +0000 (GMT) Received: from b03ledav006.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D6FB8C605A; Thu, 8 Oct 2020 12:25:36 +0000 (GMT) Received: from localhost.localdomain (unknown [9.85.87.145]) by b03ledav006.gho.boulder.ibm.com (Postfix) with ESMTP; Thu, 8 Oct 2020 12:25:36 +0000 (GMT) Subject: Re: [PATCH] perf/core: Fix hung issue on perf stat command during cpu hotplug To: 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, brho@google.com, srikar@linux.vnet.ibm.com References: <20200827064732.20860-1-kjain@linux.ibm.com> From: kajoljain Message-ID: <931cf8fd-79e9-3cbd-0943-63abea31ee8d@linux.ibm.com> Date: Thu, 8 Oct 2020 17:55:35 +0530 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: <20200827064732.20860-1-kjain@linux.ibm.com> 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=2020-10-08_07:2020-10-08,2020-10-08 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 adultscore=0 mlxscore=0 priorityscore=1501 clxscore=1011 bulkscore=0 impostorscore=0 malwarescore=0 phishscore=0 spamscore=0 mlxlogscore=999 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2010080091 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/27/20 12:17 PM, 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. Hi Peter, Please let me know if you have any comment on this patch. Thanks, Kajol Jain > > Fixes: 2ed6edd33a21("perf: Add cond_resched() to task_function_call()") > Signed-off-by: Kajol Jain > Reported-by: Srikar Dronamraju > Reviewed-by: Barret Rhoden > Tested-by: Srikar Dronamraju > --- > kernel/events/core.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > Changelog: > - Remove RFC tag > - Resolve some nits issues like space after if and > added -ENXIO in comment msg of function 'task_function_call' > as suggested by Barret Rhoden. > > Link to the RFC: https://lkml.org/lkml/2020/8/26/896 > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 5bfe8e3c6e44..cef646084198 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -99,7 +99,7 @@ static void remote_function(void *data) > * retry due to any failures in smp_call_function_single(), such as if the > * task_cpu() goes offline concurrently. > * > - * returns @func return value or -ESRCH when the process isn't running > + * returns @func return value or -ESRCH or -ENXIO when the process isn't running > */ > static int > task_function_call(struct task_struct *p, remote_function_f func, void *info) > @@ -115,7 +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; >