linux-kernel - Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU

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Date:   Sat, 7 Jan 2023 23:11:34 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     paulmck@...nel.org, Zqiang <qiang1.zhang@...el.com>,
        quic_neeraju@...cinc.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check

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.