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sched/deadline: Fix dl_server behaviour

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John reported undesirable behaviour with the dl_server since commit:
cccb45d7c4 ("sched/deadline: Less agressive dl_server handling").

When starving fair tasks on purpose (starting spinning FIFO tasks),
his fair workload, which often goes (briefly) idle, would delay fair
invocations for a second, running one invocation per second was both
unexpected and terribly slow.

The reason this happens is that when dl_se->server_pick_task() returns
NULL, indicating no runnable tasks, it would yield, pushing any later
jobs out a whole period (1 second).

Instead simply stop the server. This should restore behaviour in that
a later wakeup (which restarts the server) will be able to continue
running (subject to the CBS wakeup rules).

Notably, this does not re-introduce the behaviour cccb45d7c4 set
out to solve, any start/stop cycle is naturally throttled by the timer
period (no active cancel).

Fixes: cccb45d7c4 ("sched/deadline: Less agressive dl_server handling")
Reported-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: John Stultz <jstultz@google.com>
This commit is contained in:
Peter Zijlstra 2025-09-17 12:03:20 +02:00
parent 4ae8d9aa9f
commit a3a70caf79
3 changed files with 33 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/sched.h
View File

@ -706,7 +706,6 @@ struct sched_dl_entity {
unsigned int dl_defer : 1;
unsigned int dl_defer_armed : 1;
unsigned int dl_defer_running : 1;
unsigned int dl_server_idle : 1;
/*
* Bandwidth enforcement timer. Each -deadline task has its

23
kernel/sched/deadline.c
View File

@ -1571,10 +1571,8 @@ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p)
void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
{
/* 0 runtime = fair server disabled */
if (dl_se->dl_runtime) {
dl_se->dl_server_idle = 0;
if (dl_se->dl_runtime)
update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
}
}
void dl_server_start(struct sched_dl_entity *dl_se)
@ -1602,20 +1600,6 @@ void dl_server_stop(struct sched_dl_entity *dl_se)
dl_se->dl_server_active = 0;
}
static bool dl_server_stopped(struct sched_dl_entity *dl_se)
{
if (!dl_se->dl_server_active)
return true;
if (dl_se->dl_server_idle) {
dl_server_stop(dl_se);
return true;
}
dl_se->dl_server_idle = 1;
return false;
}
void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
dl_server_pick_f pick_task)
{
@ -2384,10 +2368,7 @@ static struct task_struct *__pick_task_dl(struct rq *rq)
if (dl_server(dl_se)) {
p = dl_se->server_pick_task(dl_se);
if (!p) {
if (!dl_server_stopped(dl_se)) {
dl_se->dl_yielded = 1;
update_curr_dl_se(rq, dl_se, 0);
}
dl_server_stop(dl_se);
goto again;
}
rq->dl_server = dl_se;

33
kernel/sched/sched.h
View File

@ -371,10 +371,39 @@ extern s64 dl_scaled_delta_exec(struct rq *rq, struct sched_dl_entity *dl_se, s6
* dl_server_update() -- called from update_curr_common(), propagates runtime
* to the server.
*
* dl_server_start()
* dl_server_stop() -- start/stop the server when it has (no) tasks.
* dl_server_start() -- start the server when it has tasks; it will stop
* automatically when there are no more tasks, per
* dl_se::server_pick() returning NULL.
*
* dl_server_stop() -- (force) stop the server; use when updating
* parameters.
*
* dl_server_init() -- initializes the server.
*
* When started the dl_server will (per dl_defer) schedule a timer for its
* zero-laxity point -- that is, unlike regular EDF tasks which run ASAP, a
* server will run at the very end of its period.
*
* This is done such that any runtime from the target class can be accounted
* against the server -- through dl_server_update() above -- such that when it
* becomes time to run, it might already be out of runtime and get deferred
* until the next period. In this case dl_server_timer() will alternate
* between defer and replenish but never actually enqueue the server.
*
* Only when the target class does not manage to exhaust the server's runtime
* (there's actualy starvation in the given period), will the dl_server get on
* the runqueue. Once queued it will pick tasks from the target class and run
* them until either its runtime is exhaused, at which point its back to
* dl_server_timer, or until there are no more tasks to run, at which point
* the dl_server stops itself.
*
* By stopping at this point the dl_server retains bandwidth, which, if a new
* task wakes up imminently (starting the server again), can be used --
* subject to CBS wakeup rules -- without having to wait for the next period.
*
* Additionally, because of the dl_defer behaviour the start/stop behaviour is
* naturally thottled to once per period, avoiding high context switch
* workloads from spamming the hrtimer program/cancel paths.
*/
extern void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec);
extern void dl_server_start(struct sched_dl_entity *dl_se);