On Tue, Feb 20, 2018 at 04:21:58PM +0100, Peter Zijlstra wrote: > On Tue, Feb 20, 2018 at 04:05:49PM +0100, Christian König wrote: > > Am 20.02.2018 um 15:54 schrieb Peter Zijlstra: > > > On Tue, Feb 20, 2018 at 03:34:07PM +0100, Christian König wrote: > > > > > OK, but neither case would in fact need the !ctx case right? That's just > > > > > there for completeness sake? > > > > Unfortunately not. TTM uses trylock to lock BOs which are about to be > > > > evicted to make room for all the BOs locked with a ctx. > > > > > > > > I need to be able to distinct between the BOs which are trylocked and those > > > > which are locked with a ctx. > > > > > > > > Writing this I actually noticed the current version is buggy, cause even > > > > when we check the mutex owner we still need to make sure that the ctx in the > > > > lock is NULL. > > > Hurm... I can't remember why trylocks behave like that, and it seems > > > rather unfortunate / inconsistent. > > > > Actually for me that is rather fortunate, cause I need to distinct between > > the locks acquired through trylock and lock. > > I suppose that would always be possible using: > ww_mutex_trylock(.ctx=NULL), and it could be that there simply weren't > any immediate uses for a !NULL trylock and it was thus not implemented. > > But that is all very long ago.. I think we simple never had a use-case for interleaving ww_mutex_lock(ctx) and ww_mutex_trylock(ctx). Nesting multiple trylocks in ctx-locks happens plenty, but not further: The common use-case for that is locking a bunch of buffers you need (for command submission or whatever), and then trylocking other buffers to make space for the buffers you need to move into VRAM. I guess if only trylocking buffers doesn't succeed in freeing up enough VRAM then we could go into blocking ww_mutex_locks which need the ctx (and which would need all the trylock-acquired buffers to be annotated with the ctx too). TTM currently tries to be far enough away from that corner case (using a defensive "never use more than 50% of all memory for gfx" approach) that it doesn't seem to need that. Once we get there it should indeed be simply to add a ctx parameter to ww_mutex_trylock to fix this case. The TTM side rework is definitely going to be the much bigger issue here ... -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
