From:   Costa Shulyupin <costa.shul@redhat.com>
To:     Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-kernel@vger.kernel.org (open list:TRACING),
        linux-trace-kernel@vger.kernel.org (open list:TRACING)
Cc:     bwensley@redhat.com, constantine.shulyupin@gmail.com,
        Costa Shulyupin <costa.shul@redhat.com>
Subject: [PATCH] tracing/hwlat: Replace sched_setaffinity with set_cpus_allowed_ptr
Date:   Thu, 16 Mar 2023 16:45:35 +0200
Message-Id: <20230316144535.1004952-1-costa.shul@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

There is a problem with the behavior of hwlat in a container,
resulting in incorrect output. A warning message is generated:
"cpumask changed while in round-robin mode, switching to mode none",
and the tracing_cpumask is ignored. This issue arises because
the kernel thread, hwlatd, is not a part of the container, and
the function sched_setaffinity is unable to locate it using its PID.
Additionally, the task_struct of hwlatd is already known.
Ultimately, the function set_cpus_allowed_ptr achieves
the same outcome as sched_setaffinity, but employs task_struct
instead of PID.

Test case:

  # cd /sys/kernel/tracing
  # echo 0 > tracing_on
  # echo round-robin > hwlat_detector/mode
  # echo hwlat > current_tracer
  # unshare --fork --pid bash -c 'echo 1 > tracing_on'
  # dmesg -c

Actual behavior:

[573502.809060] hwlat_detector: cpumask changed while in round-robin mode, switching to mode none

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
---
 kernel/trace/trace_hwlat.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index d440ddd5fd8b..444dfc31f258 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -339,7 +339,7 @@ static void move_to_next_cpu(void)
 	cpumask_clear(current_mask);
 	cpumask_set_cpu(next_cpu, current_mask);
 
-	sched_setaffinity(0, current_mask);
+	set_cpus_allowed_ptr(current, current_mask);
 	return;
 
  change_mode:
@@ -446,7 +446,7 @@ static int start_single_kthread(struct trace_array *tr)
 
 	}
 
-	sched_setaffinity(kthread->pid, current_mask);
+	set_cpus_allowed_ptr(kthread, current_mask);
 
 	kdata->kthread = kthread;
 	wake_up_process(kthread);
-- 
2.39.2