On Wed, Apr 29, 2020 at 04:41:59PM -0700, Alexei Starovoitov wrote: > On Wed, Apr 29, 2020 at 04:51:59PM -0500, Josh Poimboeuf wrote: > > On Thu, Jul 18, 2019 at 12:14:08PM -0700, tip-bot for Josh Poimboeuf wrote: > > > Commit-ID: 3193c0836f203a91bef96d88c64cccf0be090d9c > > > Gitweb: https://git.kernel.org/tip/3193c0836f203a91bef96d88c64cccf0be090d9c > > > Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > > > AuthorDate: Wed, 17 Jul 2019 20:36:45 -0500 > > > Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > > CommitDate: Thu, 18 Jul 2019 21:01:06 +0200 > > > > > > bpf: Disable GCC -fgcse optimization for ___bpf_prog_run() > > > > For some reason, this > > > > __attribute__((optimize("-fno-gcse"))) > > > > is disabling frame pointers in ___bpf_prog_run(). If you compile with > > CONFIG_FRAME_POINTER it'll show something like: > > > > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run.cold()+0x7: call without frame pointer save/setup > > you mean it started to disable frame pointers from some version of gcc? > It wasn't doing this before, since objtool wasn't complaining, right? > Sounds like gcc bug? I actually think this warning has been around for a while. I just only recently looked at it. I don't think anything changed in GCC, it's just that almost nobody uses CONFIG_FRAME_POINTER, so it wasn't really noticed. > > Also, since GCC 9.1, the GCC docs say "The optimize attribute should be > > used for debugging purposes only. It is not suitable in production > > code." That doesn't sound too promising. > > > > So it seems like this commit should be reverted. But then we're back to > > objtool being broken again in the RETPOLINE=n case, which means no ORC > > coverage in this function. (See above commit for the details) > > > > Some ideas: > > > > - Skip objtool checking of that func/file (at least for RETPOLINE=n) -- > > but then it won't have ORC coverage. > > > > - Get rid of the "double goto" in ___bpf_prog_run(), which simplifies it > > enough for objtool to understand -- but then the text explodes for > > RETPOLINE=y. > > How that will look like? > That could be the best option. For example: #define GOTO ({ goto *jumptable[insn->code]; }) and then replace all 'goto select_insn' with 'GOTO;' The problem is that with RETPOLINE=y, the function text size grows from 5k to 7k, because for each of the 160+ retpoline JMPs, GCC (stupidly) reloads the jump table register into a scratch register. > > - Add -fno-gfcse to the Makefile for kernel/bpf/core.c -- but then that > > affects the optimization of other functions in the file. However I > > don't think the impact is significant. > > > > - Move ___bpf_prog_run() to its own file with the -fno-gfcse flag. I'm > > thinking this could be the least bad option. Alexei? > > I think it would be easier to move some of the hot path > functions out of core.c instead. > Like *ksym*, BPF_CALL*, bpf_jit*, bpf_prog*. > I think resulting churn will be less. > imo it's more important to keep git blame history for interpreter > than for the other funcs. > Sounds like it's a fix that needs to be sent for the next RC ? > Please send a patch for bpf tree then. I can make a patch, what file would you recommend moving those hot path functions to? -- Josh
