From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kajol Jain Subject: [RFC] perf/core: Fixes hung issue on perf stat command during cpu hotplug Date: Wed, 26 Aug 2020 15:02:36 +0530 Message-ID: <20200826093236.446024-1-kjain@linux.ibm.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: Sender: linux-kernel-owner@vger.kernel.org To: acme@kernel.org Cc: jolsa@redhat.com, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, maddy@linux.ibm.com, peterz@infradead.org, 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, kjain@linux.ibm.com List-Id: linux-perf-users.vger.kernel.org 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 | 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(); -- 2.26.2