Re: [RFC PATCH v2 8/8] sched, smp: Trace smp callback causing an IPI

[Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>]

On 11/18/22 10:12, Peter Zijlstra wrote:
> On Thu, Nov 17, 2022 at 02:45:29PM +0000, Valentin Schneider wrote:
> 
>>> +	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 = sched_ttwu_pending;
>>> +		if (CSD_TYPE(csd) != CSD_TYPE_TTWU)
>>> +			func = csd->func;
>>> +
>>> +		if (raw_smp_call_single_queue(cpu, node))
>>> +			trace_ipi_send_cpumask(cpumask_of(cpu), _RET_IP_, func);
>> So I went with the tracepoint being placed *before* the actual IPI gets
>> sent to have a somewhat sane ordering between trace_ipi_send_cpumask() and
>> e.g. trace_call_function_single_entry().
>>
>> Packaging the call_single_queue logic makes the code less horrible, but it
>> does mix up the event ordering...
> Keeps em sharp ;-)
> 

Having the trace before the IPI avoids the (non ideal) case where the trace stops because of
an IPI execution before we have trace about who sent it... :-(.

-- Daniel