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From:   Chengming Zhou <zhouchengming@bytedance.com>
To:     hannes@cmpxchg.org, tj@kernel.org, corbet@lwn.net,
        surenb@google.com, mingo@redhat.com, peterz@infradead.org,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com
Cc:     cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, songmuchun@bytedance.com,
        Chengming Zhou <zhouchengming@bytedance.com>
Subject: [PATCH v2 02/10] sched/psi: optimize task switch inside shared cgroups again
Date:   Mon,  8 Aug 2022 19:03:33 +0800
Message-Id: <20220808110341.15799-3-zhouchengming@bytedance.com>
In-Reply-To: <20220808110341.15799-1-zhouchengming@bytedance.com>
References: <20220808110341.15799-1-zhouchengming@bytedance.com>
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commit 4117cebf1a9f ("psi: Optimize task switch inside shared cgroups")
defer prev task sleep handling to psi_task_switch(), so we don't need
to clear and set TSK_ONCPU state for common cgroups.

    A
    |
    B
   / \
  C   D
 /     \
prev   next

After that commit psi_task_switch() do:
1. psi_group_change(next, .set=TSK_ONCPU) for D
2. psi_group_change(prev, .clear=TSK_ONCPU | TSK_RUNNING) for C
3. psi_group_change(prev, .clear=TSK_RUNNING) for B, A

But there is a limitation "prev->psi_flags == next->psi_flags" that
if not satisfied, will make this cgroups optimization unusable for both
sleep switch or running switch cases. For example:

prev->in_memstall != next->in_memstall when sleep switch:
1. psi_group_change(next, .set=TSK_ONCPU) for D, B, A
2. psi_group_change(prev, .clear=TSK_ONCPU | TSK_RUNNING) for C, B, A

prev->in_memstall != next->in_memstall when running switch:
1. psi_group_change(next, .set=TSK_ONCPU) for D, B, A
2. psi_group_change(prev, .clear=TSK_ONCPU) for C, B, A

The reason why this limitation exist is that we consider a group is
PSI_MEM_FULL if the CPU is actively reclaiming and nothing productive
could run even if it were runnable. So when CPU curr changed from prev
to next and their in_memstall status is different, we have to change
PSI_MEM_FULL status for their common cgroups.

This patch remove this limitation by making psi_group_change() change
PSI_MEM_FULL status depend on CPU curr->in_memstall status.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 kernel/sched/psi.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 115a7e52fa23..9e8c5d9e585c 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -823,8 +823,6 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 	u64 now = cpu_clock(cpu);
 
 	if (next->pid) {
-		bool identical_state;
-
 		psi_flags_change(next, 0, TSK_ONCPU);
 		/*
 		 * When switching between tasks that have an identical
@@ -832,11 +830,9 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 		 * we reach the first common ancestor. Iterate @next's
 		 * ancestors only until we encounter @prev's ONCPU.
 		 */
-		identical_state = prev->psi_flags == next->psi_flags;
 		iter = NULL;
 		while ((group = iterate_groups(next, &iter))) {
-			if (identical_state &&
-			    per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
+			if (per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
 				common = group;
 				break;
 			}
@@ -883,7 +879,7 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 		 * TSK_ONCPU is handled up to the common ancestor. If we're tasked
 		 * with dequeuing too, finish that for the rest of the hierarchy.
 		 */
-		if (sleep) {
+		if (sleep || unlikely(prev->in_memstall != next->in_memstall)) {
 			clear &= ~TSK_ONCPU;
 			for (; group; group = iterate_groups(prev, &iter))
 				psi_group_change(group, cpu, clear, set, now, wake_clock);
-- 
2.36.1