Hello, On Fri, May 20, 2022 at 12:58:52AM -0700, Yosry Ahmed wrote: > On Fri, May 20, 2022 at 12:41 AM Tejun Heo <tj@xxxxxxxxxx> wrote: > > > > On Fri, May 20, 2022 at 01:21:31AM +0000, Yosry Ahmed wrote: > > > From: Hao Luo <haoluo@xxxxxxxxxx> > > > > > > Introduce a new type of iter prog: cgroup. Unlike other bpf_iter, this > > > iter doesn't iterate a set of kernel objects. Instead, it is supposed to > > > be parameterized by a cgroup id and prints only that cgroup. So one > > > needs to specify a target cgroup id when attaching this iter. The target > > > cgroup's state can be read out via a link of this iter. > > > > > > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx> > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > > > This could be me not understanding why it's structured this way but it keeps > > bothering me that this is adding a cgroup iterator which doesn't iterate > > cgroups. If all that's needed is extracting information from a specific > > cgroup, why does this need to be an iterator? e.g. why can't I use > > BPF_PROG_TEST_RUN which looks up the cgroup with the provided ID, flushes > > rstat, retrieves whatever information necessary and returns that as the > > result? > > I will let Hao and Yonghong reply here as they have a lot more > context, and they had previous discussions about cgroup_iter. I just > want to say that exposing the stats in a file is extremely convenient > for userspace apps. It becomes very similar to reading stats from > cgroupfs. It also makes migrating cgroup stats that we have > implemented in the kernel to BPF a lot easier. So, if it were upto me, I'd rather direct energy towards making retrieving information through TEST_RUN_PROG easier rather than clinging to making kernel output text. I get that text interface is familiar but it kinda sucks in many ways. > AFAIK there are also discussions about using overlayfs to have links > to the bpffs files in cgroupfs, which makes it even better. So I would > really prefer keeping the approach we have here of reading stats > through a file from userspace. As for how we go about this (and why a > cgroup iterator doesn't iterate cgroups) I will leave this for Hao and > Yonghong to explain the rationale behind it. Ideally we can keep the > same functionality under a more descriptive name/type. My answer would be the same here. You guys seem dead set on making the kernel emulate cgroup1. I'm not gonna explicitly block that but would strongly suggest having a longer term view. If you *must* do the iterator, can you at least make it a proper iterator which supports seeking? AFAICS there's nothing fundamentally preventing bpf iterators from supporting seeking. Or is it that you need something which is pinned to a cgroup so that you can emulate the directory structure? Thanks. -- tejun
