[RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

[Björn Töpel]

Mark Rutland <mark.rutland at arm.com> writes:

> Hi Bjorn
>
> (apologies, my corporate mail server has butchered your name here).

Ha! That's the price I have to pay for carrying double-umlauts
everywhere. Thanks for getting back with a really useful answer!

>> On Arm64, CALL_OPS makes it possible to implement direct calls, while
>> only patching one BL instruction -- nice!
>
> The key thing here isn't that we patch a single instruction (since we have ot
> patch the ops pointer too!); it's that we can safely patch either of the ops
> pointer or BL/NOP at any time while threads are concurrently executing.

...which indeed is a very nice property!

> If you have a multi-instruction sequence, then threads can be preempted
> mid-sequence, and it's very painful/complex to handle all of the races that
> entails.

Oh yes. RISC-V is currently using auipc/jalr with stop_machine(), and
also requires that preemtion is off. Unusable to put it blunt.

> For example, if your callsites use a sequence:
>
> 	AUIPC <tmp>, <funcptr>
> 	JALR <tmp2>, <funcptr>(<tmp>)
>
> Using stop_machine() won't allow you to patch that safely as some threads
> could be stuck mid-sequence, e.g.
>
> 	AUIPC <tmp>, <funcptr>
> 	[ preempted here ]
> 	JALR <tmp2>, <funcptr>(<tmp>)
>
> ... and you can't update the JALR to use a new funcptr immediate until those
> have completed the sequence.
>
> There are ways around that, but they're complicated and/or expensive, e.g.
>
> * Use a sequence of multiple patches, starting with replacing the JALR with an
>   exception-generating instruction with a fixup handler, which is sort-of what
>   x86 does with UD2. This may require multiple passes with
>   synchronize_rcu_tasks() to make sure all threads have seen the latest
>   instructions, and that cannot be done under stop_machine(), so if you need
>   stop_machine() for CMODx reasons, you may need to use that several times with
>   intervening calls to synchronize_rcu_tasks().
>
> * Have the patching logic manually go over each thread and fix up the pt_regs
>   for the interrupted thread. This is pretty horrid since you could have nested
>   exceptions and a task could have several pt_regs which might require
>   updating.

Yup, and both of these have rather unplesant overhead.

> The CALL_OPS approach is a bit easier to deal with as we can patch the
> per-callsite pointer atomically, then we can (possibly) enable/disable the
> callsite's branch, then wait for threads to drain once. 
>
> As a heads-up, there are some latent/generic issues with DYNAMIC_FTRACE
> generally in this area (CALL_OPs happens to side-step those, but trampoline
> usage is currently affected):
>
>   https://lore.kernel.org/lkml/Zenx_Q0UiwMbSAdP@FVFF77S0Q05N/
>
> ... I'm looking into fixing that at the moment, and it looks like that's likely
> to require some per-architecture changes.
>
>> 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.
>
> The branch range thing is annoying, but I think this boils down to the same
> problem as arm64 has with needing a "MOV <tmp>, LR" instruction that we have to
> patch in once at boot time. You could do the same and patch in the AUIPC once,
> e.g. have
>
> | 	NOP
> | 	NOP 
> | func:
> | 	AUIPC <tmp>, <common_ftrace_caller>
> | 	JALR <tmp2>, <common_ftrace_caller>(<tmp>) // patched with NOP
>
> ... which'd look very similar to arm64's sequence:
>
> | 	NOP
> | 	NOP
> | func:
> | 	MOV X9, LR
> | 	BL ftrace_caller // patched with NOP
>
> ... which I think means it *might* be better from a cmodx perspective?
>
>> 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
>> * Larger text size (function alignment + extra nops) BAD
>> * Same direct call performance NEUTRAL
>> * Same complicated text patching required NEUTRAL
>
> Is your current sequence safe for preemptible kernels (i.e. with PREEMPT_FULL=y
> or PREEMPT_DYNAMIC=y + "preempt=full" on the kernel cmdline) ?

It's very much not, and was in-fact presented by Andy (Cc) discussed at
length at Plumbers two years back.

Hmm, depending on RISC-V's CMODX path, the pro/cons CALL_OPS vs dynamic
trampolines changes quite a bit.

The more I look at the pains of patching two instruction ("split
immediates"), the better "patch data" + one insn patching look.

I which we had longer instructions, that could fit a 48b address or
more! ;-)


Again, thanks for a thought provoking reply!
Björn