[PATCH] drm: Inject a cond_resched() into long drm_clflush_sg()

[Chris Wilson]

Quoting David Laight (2020-01-16 13:58:44)
> From: Chris Wilson <chris at chris-wilson.co.uk>
> > Sent: 16 January 2020 12:29
> > 
> > Quoting David Laight (2020-01-16 12:26:45)
> > > However there is a call from __i915_gem_objet_set_pages() that
> > > is preceded by a lockdep_assert_held() check - so mustn't sleep.
> > 
> > That is a mutex; it's allowed to sleep. The might_sleep() here should
> > help assuage your fears.
> 
> Gah. Having a mutex called mm.lock isn't entirely obvious when quickly
> reading code.
> 
> From what I was reading earlier it seems that clflush() (and cflushopt)
> are fast (well not stupidly slow) for buffers under 4k.
> I suspect then can do a 'mark pending', but for larger buffers have to
> wait for earlier requests to complete (although the graph carries on
> increasing to the 16k RH margin.
> 
> If may well be that adding a cond_resched() after every 4k page
> will have ~0 performance impact because the first few clflush
> will be a bit faster (less slow).
> 
> I'll do some measurements later this afternoon.
> 
> FWIW does this code need to actually invalidate the cache lines?
> Or is it just forcing a writeback?

It is used for both.
-Chris