On Wed, Apr 05, 2023 at 01:45:02PM +0100, Valentin Schneider wrote:

On 05/04/23 14:05, Frederic Weisbecker wrote:

quoted

 static void smp_call_function_many_cond(const struct cpumask *mask,
                                      smp_call_func_t func, void *info,

@@ -946,10 +948,13 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 #endif
                      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);
-				nr_cpus++;
-				last_cpu = cpu;
-
+				if (!(scf_flags & SCF_NO_USER) ||
+				    !IS_ENABLED(CONFIG_GENERIC_ENTRY) ||
+				     ct_state_cpu(cpu) != CONTEXT_USER) {
+					__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
+					nr_cpus++;
+					last_cpu = cpu;
+				}

I've been hacking on something like this (CSD deferral for NOHZ-full),
and unfortunately this uses the CPU-local cfd_data storage thing, which
means any further smp_call_function() from the same CPU to the same
destination will spin on csd_lock_wait(), waiting for the target CPU to
come out of userspace and flush the queue - and we've just spent extra
effort into *not* disturbing it, so that'll take a while :(

I'm not sure I buy into deferring stuff.. a NOHZ_FULL cpu might 'never'
come back. Queueing data just in case it does seems wasteful.