linux-kernel - Re: [PATCH v7 4/6] rcu: Add RCU stall diagnosis information

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Message-ID: <20221114124659.GE590078@lothringen>
Date:   Mon, 14 Nov 2022 13:46:59 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, Robert Elliott <elliott@....com>
Subject: Re: [PATCH v7 4/6] rcu: Add RCU stall diagnosis information

On Mon, Nov 14, 2022 at 08:32:19PM +0800, Leizhen (ThunderTown) wrote:
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index ed93ddb8203d42c..3921aacfd421ba9 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -866,6 +866,24 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> >>  			rdp->rcu_iw_gp_seq = rnp->gp_seq;
> >>  			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
> >>  		}
> >> +
> >> +		if (rcu_cpu_stall_cputime && rdp->snap_record.gp_seq != rdp->gp_seq) {
> >> +			int cpu = rdp->cpu;
> >> +			struct rcu_snap_record *rsrp;
> >> +			struct kernel_cpustat *kcsp;
> >> +
> >> +			kcsp = &kcpustat_cpu(cpu);
> >> +
> >> +			rsrp = &rdp->snap_record;
> >> +			rsrp->cputime_irq     = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
> >> +			rsrp->cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
> >> +			rsrp->cputime_system  = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);
> >> +			rsrp->nr_hardirqs = kstat_cpu_irqs_sum(rdp->cpu);
> >> +			rsrp->nr_softirqs = kstat_cpu_softirqs_sum(rdp->cpu);
> > 
> > Getting the sum of all CPU's IRQs, with even two iterations on all of them, look
> > costly. So I have to ask: why is this information useful and why can't we deduce
> > it from other CPUs stall reports?
> 
> Only the RCU stalled CPUs are recorded. Why all CPUs?

Bah, I misread kstat_cpu_softirqs_sum() kstat_cpu_irqs_sum() content. Sorry
about that, my brainfart... :-)