On Mo, 02.05.22 19:59, Alexander Graf (graf@xxxxxxxxxx) wrote: > Lennart, looking at the current sysctl proposal, systemd could poll() on the > fork file. It would then be able to generate a /run/fork-id file which it > can use for the flow above, right? I am not to keen on making sytemd such a proxy. Sounds like something the kernel could do on its own, better and resulting in an ultimately simpler system... If systemd its the proxy this adds in extra raciness. i.e. in a ideal world, if we have some form of notification fd, then it would be great if that fd is guaranteed to have POLLIN set and its contents updated the instant the clone happened. But if we proxy this through userspace, there's necessarily a latency involved that it it takes userspace to catch up and effect the POLLIN and updated contents towards its client apps. I understand the underlying VM hypervisor APIs currently are designed to always imply some notification latency. Which sucks, but I think we should be very careful with replicating this design mistake with any userspace APIs we add. i.e. I am pretty sure that even if the underlying VM hypervisor primitive isn't as good as we wanted, the Linux kernel→userspace API should be built so that if one day a better VM hypervisor interface exists it can be plugged behind it without such limitations. > Overall, it sounds to me like the sysctl poll based kernel interface in this > patch in combination with systemd inhibitors gives us an answer to most of > the flows above. As mentioned earlier, I am not convinced sysctl is the right place for this. sysctls are understood by most people as being the place for tweaking kernel settings. This is not a kernel setting, but a notification concept, and the way Jason defined it there's nothing to read nor write, which strongly suggests to move it elsewhere, but not /proc/sys/. Use /sys/kernel/ or so. Or maybe O_NOTIFICATION_PIPE or whatever, but /proc/sys/ looks really wrong. > I can see attractiveness in providing the /run/fork-id directly from the > kernel though, to remove the dependency on systemd for poll-less > notification of libraries. I agree. Lennart -- Lennart Poettering, Berlin
