The following commit has been merged into the core/core branch of tip:
Commit-ID: a20d6f63dbfc176697886d7709312ad0a795648e
Gitweb: https://git.kernel.org/tip/a20d6f63dbfc176697886d7709312ad0a795648e
Author: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate: Thu, 03 Aug 2023 12:09:31 +02:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 Sep 2023 22:08:29 +02:00
signal: Add a proper comment about preempt_disable() in ptrace_stop()
Commit 53da1d9456fe7 ("fix ptrace slowness") added a preempt-disable section
between read_unlock() and the following schedule() invocation without
explaining why it is needed.
Replace the existing contentless comment with a proper explanation to
clarify that it is not needed for correctness but for performance reasons.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@xxxxxxxxxxxxx
---
kernel/signal.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c
index 0901901..3035beb 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2329,10 +2329,22 @@ static int ptrace_stop(int exit_code, int why, unsigned long message,
do_notify_parent_cldstop(current, false, why);
/*
- * Don't want to allow preemption here, because
- * sys_ptrace() needs this task to be inactive.
+ * The previous do_notify_parent_cldstop() invocation woke ptracer.
+ * One a PREEMPTION kernel this can result in preemption requirement
+ * which will be fulfilled after read_unlock() and the ptracer will be
+ * put on the CPU.
+ * The ptracer is in wait_task_inactive(, __TASK_TRACED) waiting for
+ * this task wait in schedule(). If this task gets preempted then it
+ * remains enqueued on the runqueue. The ptracer will observe this and
+ * then sleep for a delay of one HZ tick. In the meantime this task
+ * gets scheduled, enters schedule() and will wait for the ptracer.
*
- * XXX: implement read_unlock_no_resched().
+ * This preemption point is not bad from a correctness point of
+ * view but extends the runtime by one HZ tick time due to the
+ * ptracer's sleep. The preempt-disable section ensures that there
+ * will be no preemption between unlock and schedule() and so
+ * improving the performance since the ptracer will observe that
+ * the tracee is scheduled out once it gets on the CPU.
*/
preempt_disable();
read_unlock(&tasklist_lock);
|