Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230721121534.44328-1-ppbuk5246@gmail.com> <05e98227-77f4-4918-8f8e-2170a158e350@paulmck-laptop>
 <CAM7-yPRc4Z0siaiWf+fK2GEfjPMq5UaY13F4o19rU6NNUS5fNg@mail.gmail.com>
 <faf07eef-0a51-49f3-be48-0433952171ad@paulmck-laptop> <CAM7-yPTrHN1xPXWjSUrJeTEOuy78DpmL8ytUY+a4ZOekiAbnZg@mail.gmail.com>
 <31070aab-8665-44c4-8950-0631a777ef44@paulmck-laptop>
In-Reply-To: <31070aab-8665-44c4-8950-0631a777ef44@paulmck-laptop>
From:   Yun Levi <ppbuk5246@gmail.com>
Date:   Sun, 23 Jul 2023 07:23:00 +0100
Message-ID: <CAM7-yPSn8ietAJ8NKb0-VHDQhkHs73u--KFmO3rpTPUvcFuMvA@mail.gmail.com>
Subject: Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on rcu_report_qs_rdp
To:     paulmck@kernel.org
Cc:     frederic@kernel.org, quic_neeraju@quicinc.com,
        joel@joelfernandes.org, osh@joshtriplett.org, boqun.feng@gmail.com,
        rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, qiang.zhang1211@gmail.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Thanks for replying to reply Paul :)

> And try testing with CONFIG_RCU_STRICT_GRACE_PERIOD=y and CONFIG_PREEMPT=n.
> Though there might be better Kconfig options to use.  Those two come
> immediately to mind.

I've tested with this option via rcu torture.
and it doesn't report any problems.
and after commit 6d60ea03ac2d3 ("rcu: Report QS for outermost
PREEMPT=n rcu_read_unlock() for strict GPs")
it always makes cpu_no_qs.b.norm false whenever it calls
rcu_report_qs_rdp in rcu_read_unlock.

> But one critical piece is that softirq handlers, including the RCU_SOFTIRQ
> handler rcu_core_si(), can be invoked upon return from interrupts.

I think in that case, no problem because if it reports qs already,
rcu_check_quiescent_state wouldn't call rcu_report_qs_rdp again.
And if RCU_SOFTIRQ is called as soon as RCU interrupt is finished,
it seems the fastest one to notify qs to related tree.

> Another critical piece is that if a CPU is idle during any part of a
> grace period, the grace-period kthread can report a quiescent state on
> its behalf.

I think
    1) If timer interrupt is still programed,
          - when rcu_sched_clock_irq first reports qs, no problem
          - If qs is reported via grace period thread first,
note_gp_chagned in rcu interrupt
            will recognize this situation by setting core_needs_qs as false,
            it doesn't call rcu_report_qs_rdp thou cpu_no_qs.b.norm is true.

     2) If the timer interrupt isn't programmed,
          - rcu_gp_kthreaad reports its qs, no problem.

So, I think there's no problem removing
      "rdp->cpu_no_qs.b.norm" check in rcu_report_qs_rdp.
or wrap this condition check as WARN_ON_ONCE.

> Does that help?
Many thanks always :)

--------
SIncerely,
Levi.