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[203.173.222.16]) by smtp.gmail.com with ESMTPSA id 3sm10801746pjk.18.2021.10.14.21.14.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 Oct 2021 21:14:34 -0700 (PDT) From: Barry Song <21cnbao@gmail.com> To: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com Cc: linux-kernel@vger.kernel.org, linuxarm@huawei.com, yangyicong@hisilicon.com, Barry Song Subject: [PATCH] sched/fair: document the slow path and fast path in select_task_rq_fair Date: Fri, 15 Oct 2021 12:14:12 +0800 Message-Id: <20211015041412.5569-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Barry Song All people I know including myself took a long time to understand typical wake_up will always go to fast path and never go to slow path except FORK and EXEC. Vincent reminded me once in a linaro meeting and it 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. Cc: Vincent Guittot Signed-off-by: Barry Song --- kernel/sched/fair.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index f6a05d9b5443..ccf8f73dd4d1 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6951,6 +6951,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) break; } + /* + * This is usually true only for WF_EXEC and WF_FORK, for WF_TTWU + * it is almost always false as sched_domain hasn't SD_BALANCE_WAKE + * in default + */ if (tmp->flags & sd_flag) sd = tmp; else if (!want_affine) @@ -6958,7 +6963,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) } if (unlikely(sd)) { - /* Slow path */ + /* + * Slow path, usually only for WF_EXEC and WF_FORK; WF_TTWU almost + * always goes to fast path as sched_domain hasn't SD_BALANCE_WAKE + * in default + */ new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag); } else if (wake_flags & WF_TTWU) { /* XXX always ? */ /* Fast path */ -- 2.25.1