On 03/07/2023 17:12, Sebastian Andrzej Siewior wrote:
On 2023-07-03 16:30:01 [+0100], Tvrtko Ursulin wrote:
Hi,
Hi,
Atomic requirement from that commit text is likely referring to removing the
old big sleeping mutex we had in the reset path. So it looks plausible that
preempt_disable() section is not strictly needed and perhaps motivation
simply was, given those 20-50us polls on hw registers involved, to make them
happen as fast as possible and so minimize visual glitching during resets.
Although that reasoning would only apply on some hw generations, where the
irqsave spinlock is not held across the whole sequence anyway.
And I suspect those same platforms would be the annoying ones, if one simply
wanted to try without the preempt_disable section, given our wait_for_atomic
macro will complain loudly if not used from an atomic context.
It does not complain on RT, I did patch it out.
https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/0005-drm-i915-Don-t-check-for-atomic-context-on-PREEMPT_R.patch?h=linux-6.4.y-rt-patches
It does not complain when you patch out the complaint, correct. ;)
Only purpose of that complaint is to let developers now they are
needlessly using the atomic wait_for - the busy looping one intended for
very short delays. So if those wait_for_atomic are not really needed (in
code paths which are not under a spinlock) I'll try converting them to
non-atomic wait_for_us. Or so, will see.
Interesting, the link there refers to an older posting but this patch is
not included. For the other I didn't receive feedback at which point I
stopped pushing and put it on the list for later…
But I think we do have a macro for short register waits which works with
preempting enabled. I will try and cook up a patch and submit to our CI
during the week, then see what happens.
Or even moving the preempt_disable down so it just encompasses the register
write + wait. That would then be under the spinlock which is presumable okay
on RT? (Yes I know it wouldn't' solve one half of your "complaint" but lets
just entertain the idea for now.)
You can't
preempt_disable();
spin_lock();
You could
spin_lock();
preempt_disable();
But if there is no need then there is no need ;)
What I worry a bit the udelays…
Lets make it a two patch series and then see. First patch to see if we
can really get away without the top level preempt_disable, and then
second patch to see if we can get away with preemptible short sleeps too.
I guess on RT the top priority is consistent scheduling latency and not
so much potential UI latency in some edge cases? Saying that because if
waiting on the hw reset is made preemptible, _in theory_ it can prolong
the reset completion (as observed by i915), and so result in more UI
glitching than it normally would. Could be a theoretical point only
because it requires both CPU over-subscribe and GPU hangs. It could also
easily be that the reset path is only one path, and not so interesting
one even, which can cause this on RT.
Regards,
Tvrtko
