Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI

Peter Zijlstra
2023-03-22T09:55:20Z

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?