diff options
author | Tejun Heo <tj@kernel.org> | 2024-10-10 11:41:44 -1000 |
---|---|---|
committer | Tejun Heo <tj@kernel.org> | 2024-10-10 11:41:44 -1000 |
commit | b07996c7abac0fe3f70bf74b0b3f76eb7852ef5a (patch) | |
tree | 863493276c0aee7863bd7046f6b791b0de609482 /kernel/sched | |
parent | 967da578325d8539d42245d98f126f47abcc0569 (diff) |
sched_ext: Don't hold scx_tasks_lock for too long
While enabling and disabling a BPF scheduler, every task is iterated a
couple times by walking scx_tasks. Except for one, all iterations keep
holding scx_tasks_lock. On multi-socket systems under heavy rq lock
contention and high number of threads, this can can lead to RCU and other
stalls.
The following is triggered on a 2 x AMD EPYC 7642 system (192 logical CPUs)
running `stress-ng --workload 150 --workload-threads 10` with >400k idle
threads and RCU stall period reduced to 5s:
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 91-...!: (10 ticks this GP) idle=0754/1/0x4000000000000000 softirq=18204/18206 fqs=17
rcu: 186-...!: (17 ticks this GP) idle=ec54/1/0x4000000000000000 softirq=25863/25866 fqs=17
rcu: (detected by 80, t=10042 jiffies, g=89305, q=33 ncpus=192)
Sending NMI from CPU 80 to CPUs 91:
NMI backtrace for cpu 91
CPU: 91 UID: 0 PID: 284038 Comm: sched_ext_ops_h Kdump: loaded Not tainted 6.12.0-rc2-work-g6bf5681f7ee2-dirty #471
Hardware name: Supermicro Super Server/H11DSi, BIOS 2.8 12/14/2023
Sched_ext: simple (disabling+all)
RIP: 0010:queued_spin_lock_slowpath+0x17b/0x2f0
Code: 02 c0 10 03 00 83 79 08 00 75 08 f3 90 83 79 08 00 74 f8 48 8b 11 48 85 d2 74 09 0f 0d 0a eb 0a 31 d2 eb 06 31 d2 eb 02 f3 90 <8b> 07 66 85 c0 75 f7 39 d8 75 0d be 01 00 00 00 89 d8 f0 0f b1 37
RSP: 0018:ffffc9000fadfcb8 EFLAGS: 00000002
RAX: 0000000001700001 RBX: 0000000001700000 RCX: ffff88bfcaaf10c0
RDX: 0000000000000000 RSI: 0000000000000101 RDI: ffff88bfca8f0080
RBP: 0000000001700000 R08: 0000000000000090 R09: ffffffffffffffff
R10: ffff88a74761b268 R11: 0000000000000000 R12: ffff88a6b6765460
R13: ffffc9000fadfd60 R14: ffff88bfca8f0080 R15: ffff88bfcaac0000
FS: 0000000000000000(0000) GS:ffff88bfcaac0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f5c55f526a0 CR3: 0000000afd474000 CR4: 0000000000350eb0
Call Trace:
<NMI>
</NMI>
<TASK>
do_raw_spin_lock+0x9c/0xb0
task_rq_lock+0x50/0x190
scx_task_iter_next_locked+0x157/0x170
scx_ops_disable_workfn+0x2c2/0xbf0
kthread_worker_fn+0x108/0x2a0
kthread+0xeb/0x110
ret_from_fork+0x36/0x40
ret_from_fork_asm+0x1a/0x30
</TASK>
Sending NMI from CPU 80 to CPUs 186:
NMI backtrace for cpu 186
CPU: 186 UID: 0 PID: 51248 Comm: fish Kdump: loaded Not tainted 6.12.0-rc2-work-g6bf5681f7ee2-dirty #471
scx_task_iter can safely drop locks while iterating. Make
scx_task_iter_next() drop scx_tasks_lock every 32 iterations to avoid
stalls.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Diffstat (limited to 'kernel/sched')
-rw-r--r-- | kernel/sched/ext.c | 19 |
1 files changed, 17 insertions, 2 deletions
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index fc94048c9a98..cb1ab668e965 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -18,6 +18,12 @@ enum scx_consts { SCX_EXIT_DUMP_DFL_LEN = 32768, SCX_CPUPERF_ONE = SCHED_CAPACITY_SCALE, + + /* + * Iterating all tasks may take a while. Periodically drop + * scx_tasks_lock to avoid causing e.g. CSD and RCU stalls. + */ + SCX_OPS_TASK_ITER_BATCH = 32, }; enum scx_exit_kind { @@ -1273,6 +1279,7 @@ struct scx_task_iter { struct task_struct *locked; struct rq *rq; struct rq_flags rf; + u32 cnt; }; /** @@ -1301,6 +1308,7 @@ static void scx_task_iter_start(struct scx_task_iter *iter) iter->cursor = (struct sched_ext_entity){ .flags = SCX_TASK_CURSOR }; list_add(&iter->cursor.tasks_node, &scx_tasks); iter->locked = NULL; + iter->cnt = 0; } static void __scx_task_iter_rq_unlock(struct scx_task_iter *iter) @@ -1355,14 +1363,21 @@ static void scx_task_iter_stop(struct scx_task_iter *iter) * scx_task_iter_next - Next task * @iter: iterator to walk * - * Visit the next task. See scx_task_iter_start() for details. + * Visit the next task. See scx_task_iter_start() for details. Locks are dropped + * and re-acquired every %SCX_OPS_TASK_ITER_BATCH iterations to avoid causing + * stalls by holding scx_tasks_lock for too long. */ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter) { struct list_head *cursor = &iter->cursor.tasks_node; struct sched_ext_entity *pos; - lockdep_assert_held(&scx_tasks_lock); + if (!(++iter->cnt % SCX_OPS_TASK_ITER_BATCH)) { + scx_task_iter_unlock(iter); + cpu_relax(); + cond_resched(); + scx_task_iter_relock(iter); + } list_for_each_entry(pos, cursor, tasks_node) { if (&pos->tasks_node == &scx_tasks) |