On 20/05/2021 09:35, Tvrtko Ursulin wrote:
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.
CPU usage looks passable under a production kernel (non-debug). Once a
second refresh period, on a not really that loaded system (115 running
processes, 3096 open file descriptors as reported by lsof, none of which
are DRM), results in a system call heavy load:
real 0m55.348s
user 0m0.100s
sys 0m0.319s
Once per second loop is essentially along the lines of:
for each pid in /proc/<pid>:
for each fd in /proc/<pid>/fdinfo:
if fstatat(fd) is drm major:
read fdinfo text in one sweep and parse it
I'll post the quick intel_gpu_top patch for reference but string parsing
in C leaves a few things to be desired there.
Regards,
Tvrtko
