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Date:   Thu, 17 Nov 2022 15:26:48 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, Robert Elliott <elliott@hpe.com>
Subject: Re: [PATCH v7 4/6] rcu: Add RCU stall diagnosis information
Message-ID: <20221117142648.GF839309@lothringen>
References: <20221111130709.247-1-thunder.leizhen@huawei.com>
 <20221111130709.247-5-thunder.leizhen@huawei.com>
 <20221116223942.GA838972@lothringen>
 <d4f7f41c-e1ef-606f-d700-3e67059bb06d@huawei.com>
 <20221117122238.GC839309@lothringen>
 <54aa07c4-b7e3-1a0e-e6a3-444573caba16@huawei.com>
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On Thu, Nov 17, 2022 at 09:25:44PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/11/17 20:22, Frederic Weisbecker wrote:
> > On Thu, Nov 17, 2022 at 09:57:18AM +0800, Leizhen (ThunderTown) wrote:
> >>
> >>
> >> On 2022/11/17 6:39, Frederic Weisbecker wrote:
> >>> On Fri, Nov 11, 2022 at 09:07:07PM +0800, Zhen Lei wrote:
> >>>> @@ -262,6 +279,8 @@ struct rcu_data {
> >>>>  	short rcu_onl_gp_flags;		/* ->gp_flags at last online. */
> >>>>  	unsigned long last_fqs_resched;	/* Time of last rcu_resched(). */
> >>>>  	unsigned long last_sched_clock;	/* Jiffies of last rcu_sched_clock_irq(). */
> >>>> +	struct rcu_snap_record snap_record; /* Snapshot of core stats at half of */
> >>>> +					    /* the first RCU stall timeout */
> >>>
> >>> This should be under #ifdef CONFIG_RCU_CPU_STALL_CPUTIME
> >>
> >> This will not work for now because we also support boot option
> >> rcupdate.rcu_cpu_stall_cputime.
> > 
> > I'm confused. If CONFIG_RCU_CPU_STALL_CPUTIME=n then rcupdate.rcu_cpu_stall_cputime has
> > no effect, right?
> 
> No, rcupdate.rcu_cpu_stall_cputime override CONFIG_RCU_CPU_STALL_CPUTIME. Because
> the default value of CONFIG_RCU_CPU_STALL_CPUTIME is n, so in most cases, we need
> rcupdate.rcu_cpu_stall_cputime as the escape route.
> 
> If CONFIG_RCU_CPU_STALL_CPUTIME=y is default, your suggestion is more
> appropriate.

Oh ok I thought it was a support Kconfig switch.

Then please just mention that rcupdate.rcu_cpu_stall_cputime overrides
CONFIG_RCU_CPU_STALL_CPUTIME behaviour in the Kconfig help text.

Thanks.