On 19/05/2021 19:23, Daniel Vetter wrote:
On Wed, May 19, 2021 at 6:16 PM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
On 18/05/2021 10:40, Tvrtko Ursulin wrote:
On 18/05/2021 10:16, Daniel Stone wrote:
Hi,
On Tue, 18 May 2021 at 10:09, Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
I was just wondering if stat(2) and a chrdev major check would be a
solid criteria to more efficiently (compared to parsing the text
content) detect drm files while walking procfs.
Maybe I'm missing something, but is the per-PID walk actually a
measurable performance issue rather than just a bit unpleasant?
Per pid and per each open fd.
As said in the other thread what bothers me a bit in this scheme is that
the cost of obtaining GPU usage scales based on non-GPU criteria.
For use case of a top-like tool which shows all processes this is a
smaller additional cost, but then for a gpu-top like tool it is somewhat
higher.
To further expand, not only cost would scale per pid multiplies per open
fd, but to detect which of the fds are DRM I see these three options:
1) Open and parse fdinfo.
2) Name based matching ie /dev/dri/.. something.
3) Stat the symlink target and check for DRM major.
stat with symlink following should be plenty fast.
Maybe. I don't think my point about keeping the dentry cache needlessly
hot is getting through at all. On my lightly loaded desktop:
$ sudo lsof | wc -l
599551
$ sudo lsof | grep "/dev/dri/" | wc -l
1965
It's going to look up ~600k pointless dentries in every iteration. Just
to find a handful of DRM ones. Hard to say if that is better or worse
than just parsing fdinfo text for all files. Will see.
All sound quite sub-optimal to me.
Name based matching is probably the least evil on system resource usage
(Keeping the dentry cache too hot? Too many syscalls?), even though
fundamentally I don't it is the right approach.
What happens with dup(2) is another question.
We need benchmark numbers showing that on anything remotely realistic
it's an actual problem. Until we've demonstrated it's a real problem
we don't need to solve it.
Point about dup(2) is whether it is possible to distinguish the
duplicated fds in fdinfo. If a DRM client dupes, and we found two
fdinfos each saying client is using 20% GPU, we don't want to add it up
to 40%.
E.g. top with any sorting enabled also parses way more than it
displays on every update. It seems to be doing Just Fine (tm).
Ha, perceptions differ. I see it using 4-5% while building the kernel on
a Xeon server which I find quite a lot. :)
Does anyone have any feedback on the /proc/<pid>/gpu idea at all?
When we know we have a problem to solve we can take a look at solutions.
Yes I don't think the problem would be to add a better solution later,
so happy to try the fdinfo first. I am simply pointing out a fundamental
design inefficiency. Even if machines are getting faster and faster I
don't think that should be an excuse to waste more and more under the
hood, when a more efficient solution can be designed from the start.
Regards,
Tvrtko
