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[31.46.242.101]) by smtp.gmail.com with ESMTPSA id l20-20020a05600c089400b003fbe791a0e8sm1096397wmp.0.2023.09.19.23.57.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Sep 2023 23:57:02 -0700 (PDT) Sender: Ingo Molnar Date: Wed, 20 Sep 2023 08:57:00 +0200 From: Ingo Molnar To: linux-kernel@vger.kernel.org Cc: linux-tip-commits@vger.kernel.org, Sebastian Andrzej Siewior , Thomas Gleixner , Oleg Nesterov , x86@kernel.org Subject: Re: [tip: core/core] signal: Add a proper comment about preempt_disable() in ptrace_stop() Message-ID: References: <20230803100932.325870-2-bigeasy@linutronix.de> <169515461116.27769.12932058744920773528.tip-bot2@tip-bot2> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <169515461116.27769.12932058744920773528.tip-bot2@tip-bot2> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * tip-bot2 for Sebastian Andrzej Siewior wrote: > 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 > AuthorDate: Thu, 03 Aug 2023 12:09:31 +02:00 > Committer: Thomas Gleixner > 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 > Signed-off-by: Thomas Gleixner > Acked-by: Oleg Nesterov > Link: https://lore.kernel.org/r/20230803100932.325870-2-bigeasy@linutronix.de > > --- > 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); > Minor speling nits: > /* > - * 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 s/One /On > + * 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 s/improving the performance /improving performance > + * the tracee is scheduled out once it gets on the CPU. > */ > preempt_disable(); > read_unlock(&tasklist_lock); Thanks, Ingo