We badly need a way to query evicted memory usage. It's essential for investigating performance problems and it uncovered the buddy allocator disaster. Please either suggest an alternative, suggest changes, or review. We need it ASAP.

Thanks,

Marek

On Tue, Jan 10, 2023 at 11:55 AM Marek Olšák <maraeo@gmail.com> wrote:

On Tue, Jan 10, 2023 at 11:23 AM Christian König <ckoenig.leichtzumerken@gmail.com> wrote:

Am 10.01.23 um 16:28 schrieb Marek Olšák:

On Wed, Jan 4, 2023 at 9:51 AM Christian König <ckoenig.leichtzumerken@gmail.com> wrote:

Am 04.01.23 um 00:08 schrieb Marek Olšák:

I see about the access now, but did you even look at the patch?

I did look at the patch, but I haven't fully understood yet what you are trying to do here.

First and foremost, it returns the evicted size of VRAM and visible VRAM, and returns visible VRAM usage. It should be obvious which stat includes the size of another.

Because what the patch does isn't even exposed to common drm code, such as the preferred domain and visible VRAM placement, so it can't be in fdinfo right now.

Or do you even know what fdinfo contains? Because it contains nothing useful. It only has VRAM and GTT usage, which we already have in the INFO ioctl, so it has nothing that we need. We mainly need the eviction information and visible VRAM information now. Everything else is a bonus.

Well the main question is what are you trying to get from that information? The eviction list for example is completely meaningless to userspace, that stuff is only temporary and will be cleared on the next CS again.

I don't know what you mean. The returned eviction stats look correct and are stable (they don't change much). You can suggest changes if you think some numbers are not reported correctly.

 

What we could expose is the VRAM over-commit value, e.g. how much BOs which where supposed to be in VRAM are in GTT now. I think that's what you are looking for here, right?

The VRAM overcommit value is "evicted_vram".

 

Also, it's undesirable to open and parse a text file if we can just call an ioctl.

Well I see the reasoning for that, but I also see why other drivers do a lot of the stuff we have as IOCTL as separate files in sysfs, fdinfo or debugfs.

Especially repeating all the static information which were already available under sysfs in the INFO IOCTL was a design mistake as far as I can see. Just compare what AMDGPU and the KFD code is doing to what for example i915 is doing.

Same for things like debug information about a process. The fdinfo stuff can be queried from external tools (gdb, gputop, umr etc...) as well which makes that interface more preferred.

Nothing uses fdinfo in Mesa. No driver uses sysfs in Mesa except drm shims, noop drivers, and Intel for perf metrics. sysfs itself is an unusable mess for the PCIe query and is missing information.

I'm not against exposing more stuff through sysfs and fdinfo for tools, but I don't see any reason why drivers should use it (other than for slowing down queries and initialization).

That's what I'm asking: Is this for some tool or to make some driver decision based on it?

If you just want the numbers for over displaying then I think it would be better to put this into fdinfo together with the other existing stuff there.

If you want to make allocation decisions based on this then we should have that as IOCTL or even better as mmap() page between kernel and userspace. But in this case I would also calculation the numbers completely different as well.

See we have at least the following things in the kernel:
1. The eviction list in the VM.
    Those are the BOs which are currently evicted and tried to moved back in on the next CS.

2. The VRAM over commit value.
    In other words how much more VRAM than available has the application tried to allocate?

3. The visible VRAM usage by this application.

The end goal is that the eviction list will go away, e.g. we will always have stable allocations based on allocations of other applications and not constantly swap things in and out.

When you now expose the eviction list to userspace we will be stuck with this interface forever.

It's for the GALLIUM HUD.

The only missing thing is the size of all evicted VRAM allocations, and the size of all evicted visible VRAM allocations.

1. No list is exposed. Only sums of buffer sizes are exposed. Also, the eviction list has no meaning here. All lists are treated equally, and mem_type is compared with preferred_domains to determine where buffers are and where they should be.

2. I'm not interested in the overcommit value. I'm only interested in knowing the number of bytes of evicted VRAM right now. It can be as variable as the CPU load, but in practice it shouldn't be because PCIe doesn't have the bandwidth to move things quickly.

3. Yes, that's true.

Marek