blocking ops in drm_sched_cleanup_jobs()

[Steven Price]

Hi,

I hit the below splat randomly with panfrost. From what I can tell this
is a more general issue which would affect other drivers.

----8<-----
[58604.913130] ------------[ cut here ]------------
[58604.918590] WARNING: CPU: 1 PID: 1758 at kernel/sched/core.c:6556 __might_sleep+0x74/0x98
[58604.927965] do not call blocking ops when !TASK_RUNNING; state=1 set at [<0c590494>] prepare_to_wait_event+0x104/0x164
[58604.940047] Modules linked in: panfrost gpu_sched
[58604.945370] CPU: 1 PID: 1758 Comm: pan_js Not tainted 5.3.0-rc1+ #13
[58604.952500] Hardware name: Rockchip (Device Tree)
[58604.957815] [<c0111150>] (unwind_backtrace) from [<c010c99c>] (show_stack+0x10/0x14)
[58604.966521] [<c010c99c>] (show_stack) from [<c07adbb4>] (dump_stack+0x9c/0xd4)
[58604.974639] [<c07adbb4>] (dump_stack) from [<c0121da8>] (__warn+0xe8/0x104)
[58604.982462] [<c0121da8>] (__warn) from [<c0121e08>] (warn_slowpath_fmt+0x44/0x6c)
[58604.990867] [<c0121e08>] (warn_slowpath_fmt) from [<c014eccc>] (__might_sleep+0x74/0x98)
[58604.999973] [<c014eccc>] (__might_sleep) from [<c07c73d8>] (__mutex_lock+0x38/0x948)
[58605.008690] [<c07c73d8>] (__mutex_lock) from [<c07c7d00>] (mutex_lock_nested+0x18/0x20)
[58605.017841] [<c07c7d00>] (mutex_lock_nested) from [<bf00b54c>] (panfrost_gem_free_object+0x60/0x10c [panfrost])
[58605.029430] [<bf00b54c>] (panfrost_gem_free_object [panfrost]) from [<bf00cecc>] (panfrost_job_put+0x138/0x150 [panfrost])
[58605.042076] [<bf00cecc>] (panfrost_job_put [panfrost]) from [<bf00121c>] (drm_sched_cleanup_jobs+0xc8/0xe0 [gpu_sched])
[58605.054417] [<bf00121c>] (drm_sched_cleanup_jobs [gpu_sched]) from [<bf001300>] (drm_sched_main+0xcc/0x26c [gpu_sched])
[58605.066620] [<bf001300>] (drm_sched_main [gpu_sched]) from [<c0146cfc>] (kthread+0x13c/0x154)
[58605.076226] [<c0146cfc>] (kthread) from [<c01010b4>] (ret_from_fork+0x14/0x20)
[58605.084346] Exception stack(0xe959bfb0 to 0xe959bff8)
[58605.090046] bfa0:                                     00000000 00000000 00000000 00000000
[58605.099250] bfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[58605.108480] bfe0: 00000000 00000000 00000000 00000000 00000013 00000000
[58605.116210] irq event stamp: 179
[58605.119955] hardirqs last  enabled at (187): [<c017f7e4>] console_unlock+0x564/0x5c4
[58605.128935] hardirqs last disabled at (202): [<c017f308>] console_unlock+0x88/0x5c4
[58605.137788] softirqs last  enabled at (216): [<c0102334>] __do_softirq+0x18c/0x548
[58605.146543] softirqs last disabled at (227): [<c0129528>] irq_exit+0xc4/0x10c
[58605.154618] ---[ end trace f65bdbd9ea9adfc0 ]---
----8<-----

The problem is that drm_sched_main() calls drm_sched_cleanup_jobs() as
part of the condition of wait_event_interruptible:

> 		wait_event_interruptible(sched->wake_up_worker,
> 					 (drm_sched_cleanup_jobs(sched),
> 					 (!drm_sched_blocked(sched) &&
> 					  (entity = drm_sched_select_entity(sched))) ||
> 					 kthread_should_stop()));

When drm_sched_cleanup_jobs() is called *after* a wait (i.e. after
prepare_to_wait_event() has been called), then any might_sleep() will
moan loudly about it. This doesn't seem to happen often (I've only
triggered it once) because usually drm_sched_cleanup_jobs() either
doesn't sleep or does the sleeping during the first call that
wait_event_interruptible() makes (which is before the task state is set).

I don't really understand why drm_sched_cleanup_jobs() needs to be
called here, a simple change like below 'fixes' it. But I presume
there's some reason for the call being part of the
wait_event_interruptible condition. Can anyone shed light on this?

The code was introduced in commit 5918045c4ed4 ("drm/scheduler: rework job destruction")

Steve

----8<-----
diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index 9a0ee74d82dc..528f295e3a31 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -699,11 +699,12 @@ static int drm_sched_main(void *param)
 		struct drm_sched_job *sched_job;
 		struct dma_fence *fence;
 
+		drm_sched_cleanup_jobs(sched);
+
 		wait_event_interruptible(sched->wake_up_worker,
-					 (drm_sched_cleanup_jobs(sched),
 					 (!drm_sched_blocked(sched) &&
 					  (entity = drm_sched_select_entity(sched))) ||
-					 kthread_should_stop()));
+					 kthread_should_stop());
 
 		if (!entity)
 			continue;