3 ms·
> every program handle a "frame pointer signal" in userspace Yep. That's my proposal. > which doesn't account for the reality that we need to do frame unwindi
by quotemstr 3y ago
> every program handle a "frame pointer signal" in userspace
Yep. That's my proposal.
> which doesn't account for the reality that we need to do frame unwinding thousands of times a second with the least possible overhead
Yes, it does. The kernel has to return to userspace anyway at some point, and pushing a signal frame during that return is cheap. The cost of signal delivery is the entry into the kernel, and after a perf counter overflow, you've already paid that cost. Why would the actual unwinding be any faster in the kernel than in userspace?
Also, so what if a thread enters the kernel and samples the stack multiple times before returning to userspace? While in the kernel, the userspace stack cannot change --- therefore, it's sufficient to delay userspace stack collection until the kernel returns to userspace anyway.
You might ask "Don't we have to restore the signal mask after handling the profiling signal?"
Not if you don't define the signal to change the signal mask. sigreturn(2) is optional.
- rwmj 3y agoThis sounds vastly more complex already than following a linked list. You've also ignored the other cost which is getting the stack trace data out of the program. Anyway I'm keen to see your implementation and test how it works in reality.
- quotemstr 3y ago[flagged]
- jart 3y agoI propose that a frame pointer daemon be introduced too, for managing the frame pointer signals. We shall modify _start() to open up an io_uring connection to SystemD so that a program may share its .eh_frame data. That way the kernel can still unwind its stack in case apt upgrade changes the elf inode.
- quotemstr 3y agoNeither of you has identified anything technically wrong with unwinding via signal and neither of you has proposed a mechanism through which we might support semantically informative unwinding through paged-out code or interpreted languages. Sarcasm is not a technical argument.
- jart 3y agoI don't need to. Fedora and Ubuntu have already changed their policies to restore frame pointers. As far as I can tell, your proposal is no longer on the table. If you aren't willing to accept the decision, then you should at least understand that the onus is on you now to justify why things need to change.
- quotemstr 3y ago> It's such a clear and obvious win that the rest of us should have the opportunity to persuade them > I don't have to Pick one, jart.
- deleted 3y ago[deleted]
- rwmj 3y agoWe have to deal with reality if we want to measure and improve software performance today. The current reality is that frame pointers are the best choice. Brendan's article outlines a couple of possible future scenarios where we turn frame pointers off again, but they require work that is not done yet (in one case, advances in CPUs).
- loeg 3y agoYour argument would be more compelling without the swipe in the final sentence.