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421fc59cf5
[Symptom]
The fair server mechanism, which is intended to prevent fair starvation
when higher-priority tasks monopolize the CPU.
Specifically, RT tasks on the runqueue may not be scheduled as expected.
[Analysis]
The log "sched: DL replenish lagged too much" triggered.
By memory dump of dl_server:
curr = 0xFFFFFF80D6A0AC00 (
dl_server = 0xFFFFFF83CD5B1470(
dl_runtime = 0x02FAF080,
dl_deadline = 0x3B9ACA00,
dl_period = 0x3B9ACA00,
dl_bw = 0xCCCC,
dl_density = 0xCCCC,
runtime = 0x02FAF080,
deadline = 0x0000082031EB0E80,
flags = 0x0,
dl_throttled = 0x0,
dl_yielded = 0x0,
dl_non_contending = 0x0,
dl_overrun = 0x0,
dl_server = 0x1,
dl_server_active = 0x1,
dl_defer = 0x1,
dl_defer_armed = 0x0,
dl_defer_running = 0x1,
dl_timer = (
node = (
expires = 0x000008199756E700),
_softexpires = 0x000008199756E700,
function = 0xFFFFFFDB9AF44D30 = dl_task_timer,
base = 0xFFFFFF83CD5A12C0,
state = 0x0,
is_rel = 0x0,
is_soft = 0x0,
clock_update_flags = 0x4,
clock = 0x000008204A496900,
- The timer expiration time (rq->curr->dl_server->dl_timer->expires)
is already in the past, indicating the timer has expired.
- The timer state (rq->curr->dl_server->dl_timer->state) is 0.
[Suspected Root Cause]
The relevant code flow in the throttle path of
update_curr_dl_se() as follows:
dequeue_dl_entity(dl_se, 0); // the DL entity is dequeued
if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(dl_se))) {
if (dl_server(dl_se)) // timer registration fails
enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);//enqueue immediately
...
}
The failure of `start_dl_timer` is caused by attempting to register a
timer with an expiration time that is already in the past. When this
situation persists, the code repeatedly re-enqueues the DL entity
without properly replenishing or restarting the timer, resulting in RT
task may not be scheduled as expected.
[Proposed Solution]:
Instead of immediately re-enqueuing the DL entity on timer registration
failure, this change ensures the DL entity is properly replenished and
the timer is restarted, preventing RT potential starvation.
Fixes:
|
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.