linux-kernel - Re: [PATCH 3/3] kernel/smp: add more data to CSD lock debugging

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Date:   Fri, 26 Feb 2021 10:12:05 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, mhocko@...e.com,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 3/3] kernel/smp: add more data to CSD lock debugging

On Fri, Feb 26, 2021 at 05:38:44PM +0100, Peter Zijlstra wrote:
> 
> I hate all of this, but if this will finally catch the actual problem,
> we can then revert all this, so sure.

OK, I will bite...  What exactly do you hate about it?

							Thanx, Paul

> Also, I think this will conflict with the patches from Nadav that I have
> queued:
> 
>   https://lkml.kernel.org/r/20210220231712.2475218-1-namit@vmware.com
> 
> which I'll be pushing to tip/x86/mm once -rc1 happens.
> 
> On Fri, Feb 26, 2021 at 12:25:21PM +0100, Juergen Gross wrote:
> 
> > +static void __smp_call_single_queue_debug(int cpu, struct llist_node *node)
> > +{
> > +	unsigned int this_cpu = smp_processor_id();
> > +	struct cfd_seq_local *seq = this_cpu_ptr(&cfd_seq_local);
> > +	struct call_function_data *cfd = this_cpu_ptr(&cfd_data);
> > +	struct cfd_percpu *pcpu = per_cpu_ptr(cfd->pcpu, cpu);
> > +
> > +	cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
> > +	if (llist_add(node, &per_cpu(call_single_queue, cpu))) {
> > +		cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
> > +		cfd_seq_store(seq->ping, this_cpu, cpu, CFD_SEQ_PING);
> > +		send_call_function_single_ipi(cpu);
> > +		cfd_seq_store(seq->pinged, this_cpu, cpu, CFD_SEQ_PINGED);
> > +	} else {
> > +		cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
> > +	}
> > +}
> >  #else
> > +#define cfd_seq_store(var, src, dst, type)
> > +
> >  static void csd_lock_record(call_single_data_t *csd)
> >  {
> >  }
> > @@ -290,6 +396,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
> >  
> >  void __smp_call_single_queue(int cpu, struct llist_node *node)
> >  {
> > +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
> > +	if (static_branch_unlikely(&csdlock_debug_extended)) {
> > +		unsigned int type;
> > +
> > +		type = CSD_TYPE(container_of(node, call_single_data_t,
> > +					     node.llist));
> > +		if (type == CSD_TYPE_SYNC || type == CSD_TYPE_ASYNC) {
> > +			__smp_call_single_queue_debug(cpu, node);
> > +			return;
> > +		}
> > +	}
> > +#endif
> 
> This is a bit weird, might as well put it in generic_exec_single()
> because there you still know the type matches.
> 
> 
> > @@ -712,12 +840,21 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >  		csd->node.src = smp_processor_id();
> >  		csd->node.dst = cpu;
> >  #endif
> > -		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu)))
> > +		cfd_seq_store(pcpu->seq_queue, this_cpu, cpu, CFD_SEQ_QUEUE);
> > +		if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
> >  			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
> > +			cfd_seq_store(pcpu->seq_ipi, this_cpu, cpu, CFD_SEQ_IPI);
> > +		} else {
> > +			cfd_seq_store(pcpu->seq_noipi, this_cpu, cpu, CFD_SEQ_NOIPI);
> > +		}
> >  	}
> >  
> >  	/* Send a message to all CPUs in the map */
> > +	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->ping, this_cpu,
> > +		      CFD_SEQ_NOCPU, CFD_SEQ_PING);
> >  	arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
> > +	cfd_seq_store(this_cpu_ptr(&cfd_seq_local)->pinged, this_cpu,
> > +		      CFD_SEQ_NOCPU, CFD_SEQ_PINGED);
> 
> Too bad we can't share with the single case, a well.