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

[Chris Wilson]

Quoting Daniel Vetter (2020-01-16 06:52:42)
> On Wed, Jan 15, 2020 at 08:52:45PM +0000, Chris Wilson wrote:
> > Since we may try and flush the cachelines associated with large buffers
> > (an 8K framebuffer is about 128MiB, even before we try HDR), this leads
> > to unacceptably long latencies (when using a voluntary CONFIG_PREEMPT).
> > If we call cond_resched() between each sg chunk, that it about every 128
> > pages, we have a natural break point in which to check if the process
> > needs to be rescheduled. Naturally, this means that drm_clflush_sg() can
> > only be called from process context -- which is true at the moment. The
> > other clflush routines remain usable from atomic context.
> > 
> > Even though flushing large objects takes a demonstrable amount to time
> > to flush all the cachelines, clflush is still preferred over a
> > system-wide wbinvd as the latter has unpredictable latencies affecting
> > the whole system not just the local task.
> > 
> > Reported-by: David Laight <David.Laight at ACULAB.COM>
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: David Laight <David.Laight at ACULAB.COM>
> 
> The original bug report is complaining about latencies for SCHED_RT
> threads, on a system that doesn't even use CONFIG_PREEMPT. I'm not sure
> it's terribly valid to cater to that use-case - all the desktop distros
> seem a lot more reasonable. So firmly *shrug* from my side ...

Yeah, I had the same immediate response to the complaint), but otoh we've
inserted cond_resched() before when it looks like may consume entire
jiffies inside a loop. At the very minimum, we should have a
might_sleep() here and a reminder that this can be very slow (remember
byt?).
-Chris