Mark Rutland
2023-Feb-01 17:11 UTC
[PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
On Wed, Feb 01, 2023 at 08:57:27AM -0800, Josh Poimboeuf wrote:> On Wed, Feb 01, 2023 at 11:10:20AM +0000, Mark Rutland wrote: > > On Tue, Jan 31, 2023 at 08:38:32AM -0800, Josh Poimboeuf wrote: > > > On Tue, Jan 31, 2023 at 10:22:09AM +0000, Mark Rutland wrote: > > > > > Hm, it might be nice if our out-of-line static call implementation would > > > > > automatically do a static key check as part of static_call_cond() for > > > > > NULL-type static calls. > > > > > > > > > > But the best answer is probably to just add inline static calls to > > > > > arm64. Is the lack of objtool the only thing blocking that? > > > > > > > > The major issues were branch range limitations (and needing the linker to add > > > > PLTs), > > > > > > Does the compiler do the right thing (e.g., force PLT) if the branch > > > target is outside the translation unit? I'm wondering if we could for > > > example use objtool to help enforce such rules at the call site. > > > > It's the linker (rather than the compiler) that'll generate the PLT if the > > caller and callee are out of range at link time. There are a few other issues > > too (e.g. no guarnatee that the PLT isn't used by multiple distinct callers, > > CMODX patching requirements), so we'd have to generate a pseudo-PLT ourselves > > at build time with a patching-friendly code sequence. Ard had a prototype for > > that: > > > > https://lore.kernel.org/linux-arm-kernel/20211105145917.2828911-1-ardb at kernel.org/ > > > > ... but that was sufficiently painful that we went with the current static key > > approach: > > > > https://lore.kernel.org/all/20211109172408.49641-1-mark.rutland at arm.com/ > > Thanks for the background. > > Was there a reason for putting it out-of-line rather than directly in > _cond_resched()?I think that's mostly a historical accident; I'm not aware of a reaason we can't put that directly in _cond_resched(). Since we started from out-of-line static call trampolines, even the out-of-line static key check looked nicer, and I think we just never considered moving the static key check inline.> If it were inline then it wouldn't be that much different from the > static called version and I wonder if we could simplify by just using > the static key for all PREEMPT_DYNAMIC configs.That would be nice!> > > > If we knew each call-site would only call a particular function or skip the > > > > call, then we could do better (and would probably need something like objtool > > > > to NOP that out at compile time), but since we don't know the callee at build > > > > time we can't ensure we have a PLT in range when necessary. > > > > > > Unfortunately most static calls have multiple destinations. > > > > Sure, but here we're just enabling/disabling a call, which we could treat > > differently, or wrap at a different level within the scheduler code. I'm happy > > to take a look at that. > > I can try to emulate what you did for PREEMPT_DYNAMIC. I'll Cc you on > my actual patch to come soon-ish.I look forward to it! :)> > > And most don't have the option of being NULL. > > > > Oh, I was under the impression that all could be disabled/skipped, which is > > what a NULL target implied. > > I guess what I was trying to say is that if the target can be NULL, the > call site has to use static_call_cond() to not break the > !HAVE_STATIC_CALL case. But most call sites use static_call().Ah, sorry -- I had missed that we had distinct static_call_cond() and static_call() helpers. Thanks, Mark.