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Date:   Sat, 7 Jan 2023 23:11:34 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     paulmck@kernel.org, Zqiang <qiang1.zhang@intel.com>,
        quic_neeraju@quicinc.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check
Message-ID: <Y7nuFrJ3gjNzXmCz@lothringen>
References: <20221221201634.GH4001@paulmck-ThinkPad-P17-Gen-1>
 <344A2A3B-628E-467C-BBDF-11C3AB63D432@joelfernandes.org>
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On Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:
> (lost html content)

I can't find a place where the exp grace period sends an IPI to
CPUs slow to report a QS. But anyway you really need the tick to poll
periodically on the CPU to chase a quiescent state.

Now arguably it's probably only useful when CONFIG_PREEMPT_COUNT=y
and rcu_exp_handler() has interrupted a preempt-disabled or bh-disabled
section. Although rcu_exp_handler() sets TIF_RESCHED, which is handled
by preempt_enable() and local_bh_enable() when CONFIG_PREEMPT=y.
So probably it's only useful when CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n
(and there is also PREEMPT_DYNAMIC to consider).

If CONFIG_PREEMPT_COUNT=n, the tick can only report idle and user
as QS, but those are already reported explicitly on ct_kernel_exit() ->
rcu_preempt_deferred_qs().

Thanks.