8caf8cd** - "Do not balance task to a throttled cfs_rq"
   (follow-up performance fix)

All 7 commits were backported together to Linux 6.17 stable tree.

### Why This Change Is Necessary

Under the **old throttle model**: When a cfs_rq was throttled, its
entity was dequeued from the CPU's runqueue, preventing all tasks from
running. The PELT clock stopped, so updating group weights was
unnecessary and prevented by the `throttled_hierarchy()` check at line
3960.

Under the **new task-based throttle model** (introduced by commit
e1fad12dcb66b):
- Tasks in throttled hierarchies **continue running if in kernel mode**
- PELT clock **remains active** while throttled tasks still run/queue
- The `throttled_hierarchy()` check at line 3960 becomes **incorrect** -
  it prevents weight updates even though PELT is still running

**The fix**: Remove lines 3960-3961 to allow `calc_group_shares()` (line
3963) and `reweight_entity()` (line 3965) to execute, giving throttled
cfs_rq entities up-to-date weights for accurate CPU load calculation and
cross-cgroup fairness.

### Benefits and Trade-offs

**Benefits** (from commit message):
- Up-to-date weights enable accurate CPU load derivation
- Ensures fairness when multiple tasks from different cgroups run on
  same CPU
- Prevents stale weight values during extended kernel-mode execution

**Trade-offs** (acknowledged in commit):
- As Benjamin Segall noted: "the most likely correct distribution is the
  distribution we had at the time of throttle"
- May not matter much if tasks don't run in kernel mode for long periods
- Performance tuning was needed (see follow-up commit 0d4eaf8caf8cd
  which addresses hackbench regression by preventing load balancing to
  throttled cfs_rqs)

### What Problems Does This Solve?

The base task-based throttle model (e1fad12dcb66b) solves a **real
bug**: With the old model, a task holding a percpu_rwsem as reader in a
throttled cgroup couldn't run until the next period, causing:
- Writers waiting longer
- Reader build-up
- **Task hung warnings**

This specific commit ensures the new model works correctly by keeping
weight calculations accurate during kernel-mode execution of throttled
tasks.

### Risk Assessment

**Low to Medium Risk** for the following reasons:

**Mitigating factors**:
- Small code change (3 lines removed)
- Already backported to 6.17 stable by experienced maintainer
- Well-tested by multiple developers (Valentin Schneider, Chen Yu,
  Matteo Martelli, K Prateek Nayak)
- Part of thoroughly reviewed patch series linked at
  https://lore.kernel.org/r/20250829081120.806-4-ziqianlu@bytedance.com

**Risk factors**:
- Modifies core scheduler behavior in subtle ways
- Requires entire series (cannot be cherry-picked alone)
- Follow-up performance fixes needed (commit 0d4eaf8caf8cd mentions AMD
  Genoa performance degradation with hackbench that required additional
  checks)
- Affects PELT weight calculations during throttling edge cases

**No evidence of**:
- Reverts
- CVE assignments
- Major regression reports
- Security implications

### Backporting Requirements

If backporting to stable trees **without** the task-based throttle
model:

**DO NOT BACKPORT** - This commit will break things. The
`throttled_hierarchy()` check at line 3960 exists for a reason in the
old throttle model where PELT clocks stop on throttle.

If backporting to stable trees **with** the task-based throttle model:

**MUST BACKPORT** as part of the complete series:
1. Base commit e1fad12dcb66b (341 lines - major change)
2. Commits eb962f251fbba, 5b726e9bf9544, fe8d238e646e1
3. **This commit** (fcd394866e3db)
4. **Follow-up fixes** 253b3f5872419 and 0d4eaf8caf8cd

### Stable Tree Rules Compliance

- ✅ **Fixes important bugs**: Yes (task hung due to percpu_rwsem
  interactions)
- ✅ **Relatively small change**: Yes for this commit (3 lines), but
  series is large
- ✅ **Minimal side effects**: When backported with complete series
- ❌ **No major architectural changes**: No - this IS part of a major
  architectural change
- ✅ **Clear benefits**: Yes - prevents task hangs, improves fairness
- ⚠️ **Explicit stable tag**: No "Cc: stable" tag, but manually selected
  by stable maintainer
- ✅ **Minimal regression risk**: When backported with complete series
  and follow-ups

### Recommendation

**YES - Backport this commit**, with the following requirements:

1. **MUST include the entire task-based throttle series** (commits 1-7
   listed above)
2. **MUST include follow-up performance fixes** (especially
   0d4eaf8caf8cd)
3. **Target kernel version**: 6.17+ (already done) or newer LTS versions
   planning major scheduler updates
4. **Not suitable for**: Older stable trees without appetite for the
   341-line base architectural change

The fact that Sasha Levin already backported this entire series to 6.17
stable is the strongest indicator this is appropriate for stable
backporting.

 kernel/sched/fair.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8ce56a8d507f9..eea0b6571af5a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3957,9 +3957,6 @@ static void update_cfs_group(struct sched_entity *se)
 	if (!gcfs_rq || !gcfs_rq->load.weight)
 		return;
 
-	if (throttled_hierarchy(gcfs_rq))
-		return;
-
 	shares = calc_group_shares(gcfs_rq);
 	if (unlikely(se->load.weight != shares))
 		reweight_entity(cfs_rq_of(se), se, shares);
-- 
2.51.0[PATCH AUTOSEL 6.17] sched/fair: update_cfs_group() for throttled cfs_rqsSasha Levin <sashal@kernel.org> undefinedpatches@lists.linux.dev, stable@vger.kernel.org undefined undefined undefined undefined undefined undefined undefined undefined�?��E