* Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> [210908 14:03]: > On Wed, Sep 08, 2021 at 10:52:59AM -0700, Andrew Morton wrote: > > On Wed, 8 Sep 2021 10:21:18 -0700 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > Again I am ignorant on the details so if you can clarify the following > > > > it may help me and others to better understand the problem: > > > > > > > > 1. Peter's patch appears to just take the same "fallback" path > > > > that would be taken if the trylock failed. > > > > Is this really a breakage or just loss of performance ? > > > > I would expect the latter, since it is called "fallback". > > > > > > As Yonghong explained it's a user space breakage. > > > User space tooling expects build_id to be available 99.999% of the time > > > and that's what users observed in practice. > > > They've built a bunch of tools on top of this feature. > > > The data from these tools goes into various datacenter tables > > > and humans analyze it later. > > > So Peter's proposal is not acceptable. We don't want to get yelled at. > > > > > > > I'm not understanding. Peter said "this patch merely removes a > > performance tweak" and you and Yonghong said "it breaks userspace". > > These assertions are contradictory! > > Peter said: > "The only sane approach is making the vma tree lockless, but so far the > bpf people have resisted doing the right thing because they've been > allowed to get away with these atrocities. > " > which is partially true. > bpf folks didn't resist it. There is work ongoing to make it lockless. > It just takes an long time. I don't see how bpf folks can speed it up > any further. What work are you doing on a lockless vma tree? I've been working on the maple tree and would like to hear what you've come up with. > > > Please describe the expected userspace-visible change from Peter's > > patch in full detail? > > User space expects build_id to be available. Peter patch simply removes > that feature. > > > And yes, it is far preferable that we resolve this by changing BPF to > > be a better interface citizen, please. Let's put those thinking caps on? > > Just silence a lockdep as Yonghong proposed or some other way, > since it's only a lockdep issue. There is no actual breakage. > The feature was working and still works as intended.
