Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

[Guo Ren <guoren@kernel.org>]

On Fri, Mar 08, 2024 at 05:18:21PM +0800, Andy Chiu wrote:
> Hi Puranjay,
> 
> On Fri, Mar 8, 2024 at 3:53 AM Puranjay Mohan <puranjay12@gmail.com> wrote:
> >
> > Hi Björn,
> >
> > On Thu, Mar 7, 2024 at 8:27 PM Björn Töpel <bjorn@kernel.org> wrote:
> > >
> > > Puranjay!
> > >
> > > Puranjay Mohan <puranjay12@gmail.com> writes:
> > >
> > > > This patch enables support for DYNAMIC_FTRACE_WITH_CALL_OPS on RISC-V.
> > > > This allows each ftrace callsite to provide an ftrace_ops to the common
> > > > ftrace trampoline, allowing each callsite to invoke distinct tracer
> > > > functions without the need to fall back to list processing or to
> > > > allocate custom trampolines for each callsite. This significantly speeds
> > > > up cases where multiple distinct trace functions are used and callsites
> > > > are mostly traced by a single tracer.
> > > >
> > > > The idea and most of the implementation is taken from the ARM64's
> > > > implementation of the same feature. The idea is to place a pointer to
> > > > the ftrace_ops as a literal at a fixed offset from the function entry
> > > > point, which can be recovered by the common ftrace trampoline.
> > >
> > > Not really a review, but some more background; Another rationale (on-top
> > > of the improved per-call performance!) for CALL_OPS was to use it to
> > > build ftrace direct call support (which BPF uses a lot!). Mark, please
> > > correct me if I'm lying here!
> > >
> > > On Arm64, CALL_OPS makes it possible to implement direct calls, while
> > > only patching one BL instruction -- nice!
> > >
> > > On RISC-V we cannot use use the same ideas as Arm64 straight off,
> > > because the range of jal (compare to BL) is simply too short (+/-1M).
> > > So, on RISC-V we need to use a full auipc/jal pair (the text patching
> > > story is another chapter, but let's leave that aside for now). Since we
> > > have to patch multiple instructions, the cmodx situation doesn't really
> > > improve with CALL_OPS.
> > >
> > > Let's say that we continue building on your patch and implement direct
> > > calls on CALL_OPS for RISC-V as well.
> > >
> > > From Florent's commit message for direct calls:
> > >
> > >   |    There are a few cases to distinguish:
> > >   |    - If a direct call ops is the only one tracing a function:
> > >   |      - If the direct called trampoline is within the reach of a BL
> > >   |        instruction
> > >   |         -> the ftrace patchsite jumps to the trampoline
> > >   |      - Else
> > >   |         -> the ftrace patchsite jumps to the ftrace_caller trampoline which
> > >   |            reads the ops pointer in the patchsite and jumps to the direct
> > >   |            call address stored in the ops
> > >   |    - Else
> > >   |      -> the ftrace patchsite jumps to the ftrace_caller trampoline and its
> > >   |         ops literal points to ftrace_list_ops so it iterates over all
> > >   |         registered ftrace ops, including the direct call ops and calls its
> > >   |         call_direct_funcs handler which stores the direct called
> > >   |         trampoline's address in the ftrace_regs and the ftrace_caller
> > >   |         trampoline will return to that address instead of returning to the
> > >   |         traced function
> > >
> > > On RISC-V, where auipc/jalr is used, the direct called trampoline would
> > > always be reachable, and then first Else-clause would never be entered.
> > > This means the the performance for direct calls would be the same as the
> > > one we have today (i.e. no regression!).
> > >
> > > RISC-V does like x86 does (-ish) -- patch multiple instructions, long
> > > reach.
> > >
> > > Arm64 uses CALL_OPS and patch one instruction BL.
> > >
> > > Now, with this background in mind, compared to what we have today,
> > > CALL_OPS would give us (again assuming we're using it for direct calls):
> > >
> > > * Better performance for tracer per-call (faster ops lookup) GOOD
> >
> > ^ this was the only motivation for me to implement this patch.
> >
> > I don't think implementing direct calls over call ops is fruitful for
> > RISC-V because once
> > the auipc/jalr can be patched atomically, the direct call trampoline
> > is always reachable.
> 
> Yes, the auipc/jalr instruction pair can be patched atomically just as
> long as their size is naturally aligned on. However, we cannot prevent
> 2 instructions from being fetched atomically :P
There are some micro-arch optimization methods here, such as:
 - Disable interrupt when auipc retired.
 - When auipc -> auipc, the second one still could cause an
   interruption.

> 
> > Solving the atomic text patching problem would be fun!! I am eager to
> > see how it will be
> > solved.
> 
> I have a patch series to solve the atomic code patching issue, which I
> am about to respin [1]. The idea is to solve it with yet another layer
> of indirection. We add a 8-B aligned space at each function entry. The
> space is a pointer to the ftrace entry. During boot, each function
> entry code is updated to perform a load and then take the jump from
> the 8-B space. When ftrace is disabled, we patch the first 4B-aligned
> instruction to a jump so as to skip the ftrace entry.
> 
> We are still discussing with Alex to see if we have a better way to do
> it. If not then I'd update the patchset and re-send it. There's a
> pending improvement in the series to reduce complexity. The 8-B
> aligned space can be added before the function entry (just like your
> patch).
> 
> >
> > > * Larger text size (function alignment + extra nops) BAD
> > > * Same direct call performance NEUTRAL
> > > * Same complicated text patching required NEUTRAL
> > >
> > > It would be interesting to see how the per-call performance would
> > > improve on x86 with CALL_OPS! ;-)
> >
> > If I remember from Steven's talk, x86 uses dynamically allocated trampolines
> > for per callsite tracers, would CALL_OPS provide better performance than that?
> >
> > >
> > > I'm trying to wrap my head if it makes sense to have it on RISC-V, given
> > > that we're a bit different from Arm64. Does the scale tip to the GOOD
> > > side?
> > >
> > > Oh, and we really need to see performance numbers on real HW! I have a
> > > VF2 that I could try this series on.
> >
> > It would be great if you can do it :D.
> >
> > Thanks,
> > Puranjay
> >
> > _______________________________________________
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> 
> - [1] https://yhbt.net/lore/all/CAJF2gTSn3_cDYsF9D8dt-br2Wf_M8y02A09xgRq8kXi91sN3Aw@mail.gmail.com/T/
> 
> Regards,
> Andy
> 
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