6.15-stable review patch. If anyone has any objections, please let me know.
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From: kuyo chang kuyo.chang@mediatek.com
[ Upstream commit fc975cfb36393db1db517fbbe366e550bcdcff14 ]
In our testing with 6.12 based kernel on a big.LITTLE system, we were seeing instances of RT tasks being blocked from running on the LITTLE cpus for multiple seconds of time, apparently by the dl_server. This far exceeds the default configured 50ms per second runtime.
This is due to the fair dl_server runtime calculation being scaled for frequency & capacity of the cpu.
Consider the following case under a Big.LITTLE architecture: Assume the runtime is: 50,000,000 ns, and Frequency/capacity scale-invariance defined as below: Frequency scale-invariance: 100 Capacity scale-invariance: 50 First by Frequency scale-invariance, the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812 Then by capacity scale-invariance, it is further scaled to 4,882,812 * 50 >> 10 = 238,418. So it will scaled to 238,418 ns.
This smaller "accounted runtime" value is what ends up being subtracted against the fair-server's runtime for the current period. Thus after 50ms of real time, we've only accounted ~238us against the fair servers runtime. This 209:1 ratio in this example means that on the smaller cpu the fair server is allowed to continue running, blocking RT tasks, for over 10 seconds before it exhausts its supposed 50ms of runtime. And on other hardware configurations it can be even worse.
For the fair deadline_server, to prevent realtime tasks from being unexpectedly delayed, we really do want to use fixed time, and not scaled time for smaller capacity/frequency cpus. So remove the scaling from the fair server's accounting to fix this.
Fixes: a110a81c52a9 ("sched/deadline: Deferrable dl server") Suggested-by: Peter Zijlstra peterz@infradead.org Suggested-by: John Stultz jstultz@google.com Signed-off-by: kuyo chang kuyo.chang@mediatek.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Juri Lelli juri.lelli@redhat.com Acked-by: John Stultz jstultz@google.com Tested-by: John Stultz jstultz@google.com Link: https://lore.kernel.org/r/20250702021440.2594736-1-kuyo.chang@mediatek.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/deadline.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index ad45a8fea245e..89019a1408264 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1504,7 +1504,9 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 if (dl_entity_is_special(dl_se)) return;
- scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec); + scaled_delta_exec = delta_exec; + if (!dl_server(dl_se)) + scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
dl_se->runtime -= scaled_delta_exec;
@@ -1611,7 +1613,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 */ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p) { - s64 delta_exec, scaled_delta_exec; + s64 delta_exec;
if (!rq->fair_server.dl_defer) return; @@ -1624,9 +1626,7 @@ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p) if (delta_exec < 0) return;
- scaled_delta_exec = dl_scaled_delta_exec(rq, &rq->fair_server, delta_exec); - - rq->fair_server.runtime -= scaled_delta_exec; + rq->fair_server.runtime -= delta_exec;
if (rq->fair_server.runtime < 0) { rq->fair_server.dl_defer_running = 0;