[PATCH v1] sched: there is no need to call switch_mm_irqs_off when sched between two user thread.

[Ming Wang <wangming01@loongson.cn>]

When condition (prev->active_mm == next->mm && !prev->mm) is met,
the situation is as follows:

user thread -> user thread

There is not need switch_mm when sched between two user thread.
Because they share the mm_struct. This can provide better
performance when testing UnixBench.

Signed-off-by: Ming Wang <wangming01@loongson.cn>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 696c649..9d7f6fb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5099,7 +5099,8 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
 		 * case 'prev->active_mm == next->mm' through
 		 * finish_task_switch()'s mmdrop().
 		 */
-		switch_mm_irqs_off(prev->active_mm, next->mm, next);
+		if ((prev->active_mm != next->mm) || (!prev->mm))
+			switch_mm_irqs_off(prev->active_mm, next->mm, next);
 
 		if (!prev->mm) {                        // from kernel
 			/* will mmdrop() in finish_task_switch(). */
-- 
1.8.3.1