[PATCH 02/11] smp: Enable preemption early in smp_call_function_single

[Chuyi Zhou]

Now smp_call_function_single() disables preemption mainly for the following
reasons:

- To protect the per-cpu csd_data from concurrent modification by other
tasks on the current CPU in the !wait case. For the wait case,
synchronization is not a concern as on-stack csd is used.

- To prevent the remote online CPU from being offlined. Specifically, we
want to ensure that no new IPIs are queued after smpcfd_dying_cpu() has
finished.

Disabling preemption for the entire execution is unnecessary, especially
csd_lock_wait() part does not require preemption protection. This patch
enables preemption before csd_lock_wait() to reduce the preemption-disabled
critical section.

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
 kernel/smp.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index fc1f7a964616..0858553f3666 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -685,11 +685,24 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 
 	err = generic_exec_single(cpu, csd);
 
+	/*
+	 * We may block in csd_lock_wait() for a significant amount of time (e.g., if the
+	 * remote CPU has interrupts disabled). Disabling preemption throughout the entire
+	 * smp_call_function_single() impacts the scheduling latency and is unnecessary.
+	 *
+	 * - Preemption must be disabled before sending the IPI to ensure no new IPIs are
+	 * queued after smpcfd_dying_cpu() finishes.
+	 *
+	 * @csd is stack-allocated when @wait is true. No concurrent access except
+	 * from the IPI completion path, so we can re-enable preemption early
+	 * to reduce latency.
+	 *
+	 */
+	put_cpu();
+
 	if (wait)
 		csd_lock_wait(csd);
 
-	put_cpu();
-
 	return err;
 }
 EXPORT_SYMBOL(smp_call_function_single);
-- 
2.20.1