Commit 2917406c authored by Barry Song's avatar Barry Song Committed by Peter Zijlstra
Browse files

sched/fair: Document the slow path and fast path in select_task_rq_fair 



All People I know including myself took a long time to figure out that
typical wakeup will always go to fast path and never go to slow path
except WF_FORK and WF_EXEC.

Vincent reminded me once in a linaro meeting and made me understand
slow path won't happen for WF_TTWU. But my other friends repeatedly
wasted a lot of time on testing this path like me before I reminded
them.

So obviously the code needs some document.

Signed-off-by: default avatarBarry Song <song.bao.hua@hisilicon.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211016111109.5559-1-21cnbao@gmail.com
parent 014ba44e

kernel/sched/fair.c

+5 −0
Original line number Diff line number Diff line
@@ -6917,6 +6917,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) 
			break;

		}



		/*

		 * Usually only true for WF_EXEC and WF_FORK, as sched_domains

		 * usually do not have SD_BALANCE_WAKE set. That means wakeup

		 * will usually go to the fast path.

		 */

		if (tmp->flags & sd_flag)

			sd = tmp;

		else if (!want_affine)