On Fri, Mar 13, 2020 at 05:35:16PM -0700, Song Liu wrote: > Motivation (copied from 2/2): > > ======================= 8< ======================= > Currently, sysctl kernel.bpf_stats_enabled controls BPF runtime stats. > Typical userspace tools use kernel.bpf_stats_enabled as follows: > > 1. Enable kernel.bpf_stats_enabled; > 2. Check program run_time_ns; > 3. Sleep for the monitoring period; > 4. Check program run_time_ns again, calculate the difference; > 5. Disable kernel.bpf_stats_enabled. > > The problem with this approach is that only one userspace tool can toggle > this sysctl. If multiple tools toggle the sysctl at the same time, the > measurement may be inaccurate. > > To fix this problem while keep backward compatibility, introduce > /dev/bpf_stats. sysctl kernel.bpf_stats_enabled will only change the > lowest bit of the static key. /dev/bpf_stats, on the other hand, adds 2 > to the static key for each open fd. The runtime stats is enabled when > kernel.bpf_stats_enabled == 1 or there is open fd to /dev/bpf_stats. > > With /dev/bpf_stats, user space tool would have the following flow: > > 1. Open a fd to /dev/bpf_stats; > 2. Check program run_time_ns; > 3. Sleep for the monitoring period; > 4. Check program run_time_ns again, calculate the difference; > 5. Close the fd. > ======================= 8< ======================= > > 1/2 adds a few new API to jump_label. > 2/2 adds the /dev/bpf_stats and adjust kernel.bpf_stats_enabled handler. > > Please share your comments. Conceptually makes sense to me. Few comments: 1. I don't understand why +2 logic is necessary. Just do +1 for every FD and change proc_do_static_key() from doing explicit enable/disable to do +1/-1 as well on transition from 0->1 and 1->0. The handler would need to check that 1->1 and 0->0 is a nop. 2. /dev is kinda awkward. May be introduce a new bpf command that returns fd? 3. Instead of 1 and 2 tweak sysctl to do ++/-- unconditionally? Like repeated sysctl kernel.bpf_stats_enabled=1 will keep incrementing it and would need equal amount of sysctl kernel.bpf_stats_enabled=0 to get it back to zero where it will stay zero even if users keep spamming sysctl kernel.bpf_stats_enabled=0. This way current services that use sysctl will keep working as-is. Multiple services that currently collide on sysctl will magically start working without any changes to them. It is still backwards compatible.
