On 07/10/2021 09:50, Tvrtko Ursulin wrote:
On 06/10/2021 21:21, Barry Song wrote:
On Thu, Oct 7, 2021 at 2:44 AM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
Hi,
On 06/10/2021 08:58, Barry Song wrote:
On Wed, Oct 6, 2021 at 5:15 PM Wanghui (John)
<john.wanghui@xxxxxxxxxx> wrote:
HI Tvrtko
On 2021/10/4 22:36, Tvrtko Ursulin wrote:
void set_user_nice(struct task_struct *p, long nice)
{
bool queued, running;
- int old_prio;
+ int old_prio, ret;
struct rq_flags rf;
struct rq *rq;
@@ -6915,6 +6947,9 @@ void set_user_nice(struct task_struct *p,
long nice)
out_unlock:
task_rq_unlock(rq, p, &rf);
+
+ ret = atomic_notifier_call_chain(&user_nice_notifier_list,
nice, p);
+ WARN_ON_ONCE(ret != NOTIFY_DONE);
}
How about adding a new "io_nice" to task_struct，and move the call
chain to
sched_setattr/getattr, there are two benefits:
We already have an ionice for block io scheduler. hardly can this
new io_nice
be generic to all I/O. it seems the patchset is trying to link
process' nice with
GPU's scheduler, to some extent, it makes more senses than having a
common ionice because we have a lot of IO devices in the systems, we
don't
know which I/O the ionice of task_struct should be applied to.
Maybe we could have an ionice dedicated for GPU just like ionice for
CFQ
of bio/request scheduler.
Thought crossed my mind but I couldn't see the practicality of a 3rd
nice concept. I mean even to start with I struggle a bit with the
usefulness of existing ionice vs nice. Like coming up with practical
examples of usecases where it makes sense to decouple the two
priorities.
From a different angle I did think inheriting CPU nice makes sense for
GPU workloads. This is because today, and more so in the future,
computations on a same data set do flow from one to the other.
Like maybe a simple example of batch image processing where CPU decodes,
GPU does a transform and then CPU encodes. Or a different mix, doesn't
really matter, since the main point it is one computing pipeline from
users point of view.
I am on it. but I am also seeing two problems here:
1. nice is not global in linux. For example, if you have two cgroups,
cgroup A
has more quota then cgroup B. Tasks in B won't win even if it has a
lower nice.
cgroups will run proportional-weight time-based division of CPU.
2. Historically, we had dynamic nice which was adjusted based on the
average
sleep/running time; right now, we don't have dynamic nice, but virtual
time
still make tasks which sleep more preempt other tasks with the same nice
or even lower nice.
virtual time += physical time/weight by nice
so, static nice number doesn't always make sense to decide preemption.
So it seems your patch only works under some simple situation for example
no cgroups, tasks have similar sleep/running time.
Yes, I broadly agree with your assessment. Although there are plans for
adding cgroup support to i915 scheduling, I doubt as fine grained
control and exact semantics as there are on the CPU side will happen.
Mostly because the drive seems to be for more micro-controller managed
scheduling which adds further challenges in connecting the two sides
together.
But when you say it is a problem, I would characterize it more a
weakness in terms of being only a subset of possible control. It is
still richer (better?) than what currently exists and as demonstrated
with benchmarks in my cover letter it can deliver improvements in user
experience. If in the mid term future we can extend it with cgroup
support then the concept should still apply and get closer to how you
described nice works in the CPU world.
Main question in my mind is whether the idea of adding the
sched_attr/priority notifier to the kernel can be justified. Because as
mentioned before, everything apart from adjusting currently running GPU
jobs could be done purely in userspace. Stack changes would be quite
extensive and all, but that is not usually a good enough reason to put
something in the kernel. That's why it is an RFC an invitation to discuss.
Even ionice inherits from nice (see task_nice_ioprio()) so I think
argument can be made for drivers as well.
Now that I wrote this, I had a little bit of a light bulb moment. If I
abandon the idea of adjusting the priority of already submitted work
items, then I can do much of what I want purely from within the confines
of i915.
I simply add code to inherit from current task nice on every new work
item submission. This should probably bring the majority of the benefit
I measured.
Regards,
Tvrtko
