[PATCH] drm/ttm: Schedule out if possibe in bo delayed delete worker

[Christian König]

Am 09.04.20 um 15:25 schrieb Lucas Stach:
> Am Donnerstag, den 09.04.2020, 14:35 +0200 schrieb Christian König:
>> Am 09.04.20 um 03:31 schrieb xinhui pan:
>>> The delayed delete list is per device which might be very huge. And in
>>> a heavy workload test, the list might always not be empty. That will
>>> trigger any RCU stall warnings or softlockups in non-preemptible kernels
>>> Lets do schedule out if possible in that case.
>> Mhm, I'm not sure if that is actually allowed. This is called from a
>> work item and those are not really supposed to be scheduled away.
> Huh? Workitems can schedule out just fine, otherwise they would be
> horribly broken when it comes to sleeping locks.

Let me refine the sentence: Work items are not really supposed to be 
scheduled purposely. E.g. you shouldn't call schedule() or 
cond_resched() like in the case here.

Getting scheduled away because we wait for a lock is of course perfectly 
fine.

>   The workqueue code
> even has measures to keep the workqueues at the expected concurrency
> level by starting other workitems when one of them goes to sleep.

Yeah, and exactly that's what I would say we should avoid here :)

In other words work items can be scheduled away, but they should not if 
not really necessary (e.g. waiting for a lock).

Otherwise as you said new threads for work item processing are started 
up and I don't think we want that.

Just returning from the work item and waiting for the next cycle is most 
likely the better option.

Regards,
Christian.

>
> Regards,
> Lucas
>
>> Maybe rather change the while into while (!list_empty(&bdev->ddestroy)
>> && !should_reschedule(0)).
>>
>> Christian.
>>
>>> Signed-off-by: xinhui pan <xinhui.pan at amd.com>
>>> ---
>>>    drivers/gpu/drm/ttm/ttm_bo.c | 1 +
>>>    1 file changed, 1 insertion(+)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>>> index 9e07c3f75156..b8d853cab33b 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>> @@ -541,6 +541,7 @@ static bool ttm_bo_delayed_delete(struct ttm_bo_device *bdev, bool remove_all)
>>>    		}
>>>    
>>>    		ttm_bo_put(bo);
>>> +		cond_resched();
>>>    		spin_lock(&glob->lru_lock);
>>>    	}
>>>    	list_splice_tail(&removed, &bdev->ddestroy);
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel at lists.freedesktop.org
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Fmailman%2Flistinfo%2Fdri-devel&data=02%7C01%7Cchristian.koenig%40amd.com%7C0a47486676a74702f05408d7dc89839c%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637220355504145868&sdata=wbRkYBPI6mYuZjKBtQN3AGLDOwqJlWY3XUtwwSiUQHg%3D&reserved=0