On Wed, Jun 22, 2016 at 11:22 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Wed, Jun 22, 2016 at 10:59:23AM -0700, Andy Lutomirski wrote: >> > So I got a chance to look at this some more. I'm thinking that to make >> > this feature more consistently useful, we shouldn't only annotate >> > pt_regs frames for calls to handlers; other calls should be annotated as >> > well: preempt_schedule_irq, CALL_enter_from_user_mode, >> > prepare_exit_to_usermode, SWAPGS, TRACE_IRQS_OFF, DISABLE_INTERRUPTS, >> > etc. That way, the unwinder will always be able to find pt_regs from an >> > interrupt/exception, even if starting from one of these other calls. >> > >> > But then, things get ugly. You have to either setup and tear down the >> > frame for every possible call, or do a higher-level setup/teardown >> > across multiple calls, which invalidates several assumptions in the >> > entry code about the location of pt_regs on the stack. >> > >> > Also problematic is that several of the macros (like TRACE_IRQS_IRETQ) >> > make assumptions about the location of pt_regs. And they're used by >> > both syscall and interrupt code. So if we didn't create a frame pointer >> > header for syscalls, we'd basically need two versions of the macros: one >> > for irqs/exceptions and one for syscalls. >> > >> > So I think the cleanest way to handle this is to always allocate two >> > extra registers on the stack in ALLOC_PT_GPREGS_ON_STACK. Then all >> > entry code can assume that pt_regs is at a constant location, and all >> > the above problems go away. Another benefit is that we'd only need two >> > saves instead of three -- the pointer to pt_regs is no longer needed >> > since pt_regs is always immediately after the frame header. >> > >> > I worked up a patch to implement this -- see below. It writes the frame >> > pointer in all entry paths, including syscalls. This helps keep the >> > code simple. >> > >> > The downside is a small performance penalty: with getppid()-in-a-loop on >> > my laptop, the average syscall went from 52ns to 53ns, which is about a >> > 2% slowdown. But I doubt it would be measurable in a real-world >> > workload. >> > >> > It looks like about half the slowdown is due to the extra stack >> > allocation (which presumably adds a little d-cache pressure on the stack >> > memory) and the other half is due to the stack writes. >> > >> > I could remove the writes from the syscall path but it would only save >> > about half a ns, and it would make the code less robust. Plus it's nice >> > to have the consistency of having *all* pt_regs frames annotated. >> >> This is a bit messy, and I'm not really sure that the entry code >> should be have to operate under constraints like this. Also, >> convincing myself this works for NMI sounds unpleasant. >> >> Maybe we should go back to my idea of just listing the call sites in a table. > > So are you suggesting something like: > > .macro ENTRY_CALL func pt_regs_offset=0 > call \func > 1: .pushsection .entry_calls, "a" > .long 1b - . > .long \pt_regs_offset > .popsection > .endm > > and then change every call in the entry code to ENTRY_CALL? Yes, exactly, modulo whether the section name is good. hpa is probably the authority on that. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
