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Date: Wed, 22 Mar 2023 10:53:29 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <vschneid@redhat.com>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
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On Tue, Mar 07, 2023 at 02:35:58PM +0000, Valentin Schneider wrote:

> @@ -477,6 +490,25 @@ static __always_inline void csd_unlock(struct __call_single_data *csd)
>  	smp_store_release(&csd->node.u_flags, 0);
>  }
>  
> +static __always_inline void
> +raw_smp_call_single_queue(int cpu, struct llist_node *node, smp_call_func_t func)
> +{
> +	/*
> +	 * The list addition should be visible to the target CPU when it pops
> +	 * the head of the list to pull the entry off it in the IPI handler
> +	 * because of normal cache coherency rules implied by the underlying
> +	 * llist ops.
> +	 *
> +	 * If IPIs can go out of order to the cache coherency protocol
> +	 * in an architecture, sufficient synchronisation should be added
> +	 * to arch code to make it appear to obey cache coherency WRT
> +	 * locking and barrier primitives. Generic code isn't really
> +	 * equipped to do the right thing...
> +	 */
> +	if (llist_add(node, &per_cpu(call_single_queue, cpu)))
> +		send_call_function_single_ipi(cpu, func);
> +}
> +
>  static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
>  
>  void __smp_call_single_queue(int cpu, struct llist_node *node)
> @@ -493,21 +525,25 @@ void __smp_call_single_queue(int cpu, struct llist_node *node)
>  		}
>  	}
>  #endif
>  	/*
> +	 * We have to check the type of the CSD before queueing it, because
> +	 * once queued it can have its flags cleared by
> +	 *   flush_smp_call_function_queue()
> +	 * even if we haven't sent the smp_call IPI yet (e.g. the stopper
> +	 * executes migration_cpu_stop() on the remote CPU).
>  	 */
> +	if (trace_ipi_send_cpumask_enabled()) {
> +		call_single_data_t *csd;
> +		smp_call_func_t func;
> +
> +		csd = container_of(node, call_single_data_t, node.llist);
> +		func = CSD_TYPE(csd) == CSD_TYPE_TTWU ?
> +			sched_ttwu_pending : csd->func;
> +
> +		raw_smp_call_single_queue(cpu, node, func);
> +	} else {
> +		raw_smp_call_single_queue(cpu, node, NULL);
> +	}
>  }

Hurmph... so we only really consume @func when we IPI. Would it not be
more useful to trace this thing for *every* csd enqeued?