On Wed, May 03, 2023 at 06:35:49AM -1000, Tejun Heo wrote: > Hello, Kent. > > On Wed, May 03, 2023 at 04:05:08AM -0400, Kent Overstreet wrote: > > No, we're still waiting on the tracing people to _demonstrate_, not > > claim, that this is at all possible in a comparable way with tracing. > > So, we (meta) happen to do stuff like this all the time in the fleet to hunt > down tricky persistent problems like memory leaks, ref leaks, what-have-you. > In recent kernels, with kprobe and BPF, our ability to debug these sorts of > problems has improved a great deal. Below, I'm attaching a bcc script I used > to hunt down, IIRC, a double vfree. It's not exactly for a leak but leaks > can follow the same pattern. > > There are of course some pros and cons to this approach: > > Pros: > > * The framework doesn't really have any runtime overhead, so we can have it > deployed in the entire fleet and debug wherever problem is. > > * It's fully flexible and programmable which enables non-trivial filtering > and summarizing to be done inside kernel w/ BPF as necessary, which is > pretty handy for tracking high frequency events. > > * BPF is pretty performant. Dedicated built-in kernel code can do better of > course but BPF's jit compiled code & its data structures are fast enough. > I don't remember any time this was a problem. > > Cons: > > * BPF has some learning curve. Also the fact that what it provides is a wide > open field rather than something scoped out for a specific problem can > make it seem a bit daunting at the beginning. > > * Because tracking starts when the script starts running, it doesn't know > anything which has happened upto that point, so you gotta pay attention to > handling e.g. handling frees which don't match allocs. It's kinda annoying > but not a huge problem usually. There are ways to build in BPF progs into > the kernel and load it early but I haven't experiemnted with it yet > personally. > > I'm not necessarily against adding dedicated memory debugging mechanism but > do wonder whether the extra benefits would be enough to justify the code and > maintenance overhead. > > Oh, a bit of delta but for anyone who's more interested in debugging > problems like this, while I tend to go for bcc > (https://github.com/iovisor/bcc) for this sort of problems. Others prefer to > write against libbpf directly or use bpftrace > (https://github.com/iovisor/bpftrace). Do you have example output? TBH I'm skeptical that it's even possible to do full memory allocation profiling with tracing/bpf, due to recursive memory allocations and needing an index of outstanding allcations.
