Hello,
While debugging a softlock that happens on an ioctl(RADEON_CS), I found that it keeps looping indefinitely in the following loop:
That would be great if someone could explain the logic behind this loop iteration. My understanding is that it tries to get a free node to put the current buffer object by calling "ttm_bo_man_get_node". If it fails with mem->mm_node as NULL (internally -ENOSPC) then it tries to evict another buffer from the LRU by calling "ttm_mem_evict_first". And then try again (until ?).
For some reasons, after some points while running an app that GL upload a lot of images, these 2 functions keeps returning 0 with mem->mm_node as NULL so the "while (true)" keeps looping indefinitely. Which results in the process to be stuck in that ioctl for ever.
A nasty workaround is to break the loop after a threshold for the number of iterations. It looks like it very rarely goes over 200. So breaking if > 200 iteration and returning -ENOMEM allows the application to get the hand back instead of being stuck. This is quite helpful for the debugging phase but definitely not a proper fix.
A colleague found that changing ttm_bo_unreserve by __ttm_bo_unreserve here https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/ttm/ttm_bo.c#L751 fixes this softlock. Because the later does not re-add the evicted buffer to the LRU.
But we are unsure whether this is a proper fix or just a workaround, providing this line exists since the first TTM commit in 2009. Any comment ?
Also it looks like there is a recursion from:
radeon_cs_ioctl
radeon_cs_parser_relocs
radeon_bo_list_validate
ttm_bo_validate
ttm_bo_move_buffer
ttm_bo_mem_space @
ttm_bo_mem_force_space
ttm_mem_evict_first
ttm_bo_evict
ttm_bo_mem_space @
ttm_mem_evict_first
...
It looks it is meant to work like this but this make it complicated to follow. So any input would be much appreciated. Especially about the eviction mechanism + bo->evicted flag and how TTM manages the LRU for corner cases like when the VRAM is full.
I tried kernel 4.4, 4.8 and git HEAD from last week.
Thx
Julien
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