Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

From: Puranjay Mohan
Date: Wed Mar 06 2024 - 19:17:40 EST

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Hi Alex,

On Wed, Mar 6, 2024 at 9:35 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> Hi Puranjay,
>
> On 06/03/2024 17:59, Puranjay Mohan wrote:
> > 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.
> >
> > We use -fpatchable-function-entry to reserve 8 bytes above the function
> > entry by emitting 2 4 byte or 4 2 byte nops depending on the presence of
> > CONFIG_RISCV_ISA_C. These 8 bytes are patched at runtime with a pointer
> > to the associated ftrace_ops for that callsite. Functions are aligned to
> > 8 bytes to make sure that the accesses to this literal are atomic.
> >
> > This approach allows for directly invoking ftrace_ops::func even for
> > ftrace_ops which are dynamically-allocated (or part of a module),
> > without going via ftrace_ops_list_func.
> >
> > I've benchamrked this with the ftrace_ops sample module on Qemu, with
> > the next version, I will provide benchmarks on real hardware:
> >
> > Without this patch:
> >
> > +-----------------------+-----------------+----------------------------+
> > | Number of tracers | Total time (ns) | Per-call average time |
> > |-----------------------+-----------------+----------------------------|
> > | Relevant | Irrelevant | 100000 calls | Total (ns) | Overhead (ns) |
> > |----------+------------+-----------------+------------+---------------|
> > | 0 | 0 | 15615700 | 156 | - |
> > | 0 | 1 | 15917600 | 159 | - |
> > | 0 | 2 | 15668000 | 156 | - |
> > | 0 | 10 | 14971500 | 149 | - |
> > | 0 | 100 | 15417600 | 154 | - |
> > | 0 | 200 | 15387000 | 153 | - |
> > |----------+------------+-----------------+------------+---------------|
> > | 1 | 0 | 119906800 | 1199 | 1043 |
> > | 1 | 1 | 137428600 | 1374 | 1218 |
> > | 1 | 2 | 159562400 | 1374 | 1218 |
> > | 1 | 10 | 302099900 | 3020 | 2864 |
> > | 1 | 100 | 2008785500 | 20087 | 19931 |
> > | 1 | 200 | 3965221900 | 39652 | 39496 |
> > |----------+------------+-----------------+------------+---------------|
> > | 1 | 0 | 119166700 | 1191 | 1035 |
> > | 2 | 0 | 157999900 | 1579 | 1423 |
> > | 10 | 0 | 425370100 | 4253 | 4097 |
> > | 100 | 0 | 3595252100 | 35952 | 35796 |
> > | 200 | 0 | 7023485700 | 70234 | 70078 |
> > +----------+------------+-----------------+------------+---------------+
> >
> > Note: per-call overhead is estimated relative to the baseline case with
> > 0 relevant tracers and 0 irrelevant tracers.
> >
> > With this patch:
> >
> > +-----------------------+-----------------+----------------------------+
> > | Number of tracers | Total time (ns) | Per-call average time |
> > |-----------------------+-----------------+----------------------------|
> > | Relevant | Irrelevant | 100000 calls | Total (ns) | Overhead (ns) |
> > |----------+------------+-----------------+------------+---------------|
> > | 0 | 0 | 15254600 | 152 | - |
> > | 0 | 1 | 16136700 | 161 | - |
> > | 0 | 2 | 15329500 | 153 | - |
> > | 0 | 10 | 15148800 | 151 | - |
> > | 0 | 100 | 15746900 | 157 | - |
> > | 0 | 200 | 15737400 | 157 | - |
> > |----------+------------+-----------------+------------+---------------|
> > | 1 | 0 | 47909000 | 479 | 327 |
> > | 1 | 1 | 48297400 | 482 | 330 |
> > | 1 | 2 | 47314100 | 473 | 321 |
> > | 1 | 10 | 47844900 | 478 | 326 |
> > | 1 | 100 | 46591900 | 465 | 313 |
> > | 1 | 200 | 47178900 | 471 | 319 |
> > |----------+------------+-----------------+------------+---------------|
> > | 1 | 0 | 46715800 | 467 | 315 |
> > | 2 | 0 | 155134500 | 1551 | 1399 |
> > | 10 | 0 | 442672800 | 4426 | 4274 |
> > | 100 | 0 | 4092353900 | 40923 | 40771 |
> > | 200 | 0 | 7135796400 | 71357 | 71205 |
> > +----------+------------+-----------------+------------+---------------+
> >
> > Note: per-call overhead is estimated relative to the baseline case with
> > 0 relevant tracers and 0 irrelevant tracers.
> >
> > As can be seen from the above:
> >
> > a) Whenever there is a single relevant tracer function associated with a
> > tracee, the overhead of invoking the tracer is constant, and does not
> > scale with the number of tracers which are *not* associated with that
> > tracee.
> >
> > b) The overhead for a single relevant tracer has dropped to ~1/3 of the
> > overhead prior to this series (from 1035ns to 315ns). This is largely
> > due to permitting calls to dynamically-allocated ftrace_ops without
> > going through ftrace_ops_list_func.
> >
> > Why is this patch a RFC patch:
> > 1. I saw some rcu stalls on Qemu and need to debug them and see if they
> > were introduced by this patch.
>
>
> FYI, I'm currently working on debugging such issues (and other) with the
> *current* ftrace implementation, so probably not caused by your
> patchset. But keep debugging too, maybe this introduces other issues or
> even better, you'll find the root cause :)
>
>
> > 2. This needs to be tested thoroughly on real hardware.
> > 3. Seeking reviews to fix any fundamental problems with this patch that I
> > might have missed due to my lack of RISC-V architecture knowledge.
> > 4. I would like to benchmark this on real hardware and put the results in
> > the commit message.
> >
> > Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> > ---
> > arch/riscv/Kconfig | 2 ++
> > arch/riscv/Makefile | 8 +++++
> > arch/riscv/include/asm/ftrace.h | 3 ++
> > arch/riscv/kernel/asm-offsets.c | 3 ++
> > arch/riscv/kernel/ftrace.c | 59 +++++++++++++++++++++++++++++++++
> > arch/riscv/kernel/mcount-dyn.S | 42 ++++++++++++++++++++---
> > 6 files changed, 112 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 0bfcfec67ed5..e474742e23b2 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -78,6 +78,7 @@ config RISCV
> > select EDAC_SUPPORT
> > select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRACE)
> > select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY if DYNAMIC_FTRACE
> > + select FUNCTION_ALIGNMENT_8B if DYNAMIC_FTRACE_WITH_CALL_OPS
>
>
> A recent discussion [1] states that -falign-functions cannot guarantee
> this alignment for all code and that gcc developers came up with a new
> option [2]: WDYT? I have added Andy and Evgenii in +cc to help on that.

I saw arm64 uses the same and assumes this guarantee, maybe it is broken too?
Will look into the discussion and try to use the other option. I am
currently using Clang to build the kernel.

>
> [1]
> https://lore.kernel.org/linux-riscv/4fe4567b-96be-4102-952b-7d39109b2186@xxxxxxxxx/
> [2]
> https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=0f5a9a00e3ab1fe96142f304cfbcf3f63b15f326
>
>
> > select GENERIC_ARCH_TOPOLOGY
> > select GENERIC_ATOMIC64 if !64BIT
> > select GENERIC_CLOCKEVENTS_BROADCAST if SMP
> > @@ -127,6 +128,7 @@ config RISCV
> > select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
> > select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> > select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> > + select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if (DYNAMIC_FTRACE_WITH_REGS && !CFI_CLANG)
> > select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> > select HAVE_FUNCTION_GRAPH_TRACER
> > select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index 252d63942f34..875ad5dc3d32 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -14,12 +14,20 @@ endif
> > ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
> > LDFLAGS_vmlinux += --no-relax
> > KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> > +ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS), y)
> > +ifeq ($(CONFIG_RISCV_ISA_C),y)
> > + CC_FLAGS_FTRACE := -fpatchable-function-entry=8,4
> > +else
> > + CC_FLAGS_FTRACE := -fpatchable-function-entry=4,2
> > +endif
> > +else
> > ifeq ($(CONFIG_RISCV_ISA_C),y)
> > CC_FLAGS_FTRACE := -fpatchable-function-entry=4
> > else
> > CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> > endif
> > endif
> > +endif
> >
> > ifeq ($(CONFIG_CMODEL_MEDLOW),y)
> > KBUILD_CFLAGS_MODULE += -mcmodel=medany
> > diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> > index 329172122952..c9a84222c9ea 100644
> > --- a/arch/riscv/include/asm/ftrace.h
> > +++ b/arch/riscv/include/asm/ftrace.h
> > @@ -28,6 +28,9 @@
> > void MCOUNT_NAME(void);
> > static inline unsigned long ftrace_call_adjust(unsigned long addr)
> > {
> > + if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
> > + return addr + 8;
> > +
> > return addr;
> > }
> >
> > diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> > index a03129f40c46..7d7c4b486852 100644
> > --- a/arch/riscv/kernel/asm-offsets.c
> > +++ b/arch/riscv/kernel/asm-offsets.c
> > @@ -488,4 +488,7 @@ void asm_offsets(void)
> > DEFINE(STACKFRAME_SIZE_ON_STACK, ALIGN(sizeof(struct stackframe), STACK_ALIGN));
> > OFFSET(STACKFRAME_FP, stackframe, fp);
> > OFFSET(STACKFRAME_RA, stackframe, ra);
> > +#ifdef CONFIG_FUNCTION_TRACER
> > + DEFINE(FTRACE_OPS_FUNC, offsetof(struct ftrace_ops, func));
> > +#endif
> > }
> > diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> > index f5aa24d9e1c1..e2e75e15d32e 100644
> > --- a/arch/riscv/kernel/ftrace.c
> > +++ b/arch/riscv/kernel/ftrace.c
> > @@ -82,9 +82,52 @@ static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
> > return 0;
> > }
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > +static const struct ftrace_ops *riscv64_rec_get_ops(struct dyn_ftrace *rec)
> > +{
> > + const struct ftrace_ops *ops = NULL;
> > +
> > + if (rec->flags & FTRACE_FL_CALL_OPS_EN) {
> > + ops = ftrace_find_unique_ops(rec);
> > + WARN_ON_ONCE(!ops);
> > + }
> > +
> > + if (!ops)
> > + ops = &ftrace_list_ops;
> > +
> > + return ops;
> > +}
> > +
> > +static int ftrace_rec_set_ops(const struct dyn_ftrace *rec,
> > + const struct ftrace_ops *ops)
> > +{
> > + unsigned long literal = rec->ip - 8;
> > +
> > + return patch_text_nosync((void *)literal, &ops, sizeof(ops));

^^
I will change this to use a new function that does a 64 bit write and
doesn't do flush_icache_range() as naturally aligned writes are
atomic and therefore synchronization is not required here.

> > +}
> > +
> > +static int ftrace_rec_set_nop_ops(struct dyn_ftrace *rec)
> > +{
> > + return ftrace_rec_set_ops(rec, &ftrace_nop_ops);
> > +}
> > +
> > +static int ftrace_rec_update_ops(struct dyn_ftrace *rec)
> > +{
> > + return ftrace_rec_set_ops(rec, riscv64_rec_get_ops(rec));
> > +}
> > +#else
> > +static int ftrace_rec_set_nop_ops(struct dyn_ftrace *rec) { return 0; }
> > +static int ftrace_rec_update_ops(struct dyn_ftrace *rec) { return 0; }
> > +#endif
> > +
> > int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> > {
> > unsigned int call[2];
> > + int ret;
> > +
> > + ret = ftrace_rec_update_ops(rec);
> > + if (ret)
> > + return ret;
> >
> > make_call_t0(rec->ip, addr, call);
> >
> > @@ -98,6 +141,11 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> > unsigned long addr)
> > {
> > unsigned int nops[2] = {NOP4, NOP4};
> > + int ret;
> > +
> > + ret = ftrace_rec_set_nop_ops(rec);
> > + if (ret)
> > + return ret;
> >
> > if (patch_text_nosync((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
> > return -EPERM;
> > @@ -125,6 +173,13 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> >
> > int ftrace_update_ftrace_func(ftrace_func_t func)
> > {
> > + /*
> > + * When using CALL_OPS, the function to call is associated with the
> > + * call site, and we don't have a global function pointer to update.
> > + */
> > + if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
> > + return 0;
> > +
> > int ret = __ftrace_modify_call((unsigned long)&ftrace_call,
> > (unsigned long)func, true, true);
> > if (!ret) {
> > @@ -147,6 +202,10 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> > make_call_t0(caller, old_addr, call);
> > ret = ftrace_check_current_call(caller, call);
> >
> > + if (ret)
> > + return ret;
> > +
> > + ret = ftrace_rec_update_ops(rec);
> > if (ret)
> > return ret;
> >
> > diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
> > index b7561288e8da..cb241e36e514 100644
> > --- a/arch/riscv/kernel/mcount-dyn.S
> > +++ b/arch/riscv/kernel/mcount-dyn.S
> > @@ -191,11 +191,35 @@
> > .endm
> >
> > .macro PREPARE_ARGS
> > - addi a0, t0, -FENTRY_RA_OFFSET
> > + addi a0, t0, -FENTRY_RA_OFFSET // ip (callsite's auipc insn)
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > + /*
> > + * When CALL_OPS is enabled (2 or 4) nops [8B] are placed before the
> > + * function entry, these are later overwritten with the pointer to the
> > + * associated struct ftrace_ops.
> > + *
> > + * -8: &ftrace_ops of the associated tracer function.
> > + *<ftrace enable>:
> > + * 0: auipc t0/ra, 0x?
> > + * 4: jalr t0/ra, ?(t0/ra)
> > + *
> > + * -8: &ftrace_nop_ops
> > + *<ftrace disable>:
> > + * 0: nop
> > + * 4: nop
> > + *
> > + * t0 is set to ip+8 after the jalr is executed at the callsite,
> > + * so we find the associated op at t0-16.
> > + */
> > + mv a1, ra // parent_ip
> > + REG_L a2, -16(t0) // op
> > + REG_L ra, FTRACE_OPS_FUNC(a2) // op->func
> > +#else
> > la a1, function_trace_op
> > - REG_L a2, 0(a1)
> > - mv a1, ra
> > - mv a3, sp
> > + REG_L a2, 0(a1) // op
> > + mv a1, ra // parent_ip
> > +#endif
> > + mv a3, sp // regs
> > .endm
> >
> > #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> > @@ -233,8 +257,12 @@ SYM_FUNC_START(ftrace_regs_caller)
> > SAVE_ABI_REGS 1
> > PREPARE_ARGS
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > + jalr ra
> > +#else
> > SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
> > call ftrace_stub
> > +#endif
> >
> > RESTORE_ABI_REGS 1
> > bnez t1, .Ldirect
> > @@ -247,9 +275,13 @@ SYM_FUNC_START(ftrace_caller)
> > SAVE_ABI_REGS 0
> > PREPARE_ARGS
> >
> > -SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)

^^ this hunk is a mistake, I will fix it in the next version.

> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > + jalr ra
> > +#else
> > +SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
> > call ftrace_stub
> >
> > +#endif
> > RESTORE_ABI_REGS 0
> > jr t0
> > SYM_FUNC_END(ftrace_caller)
>
>
> As I'm diving into ftrace right now, I'll give a proper review soon. But
> as a note, I noticed that the function_graph tracer, when enabled, makes
> the whole system unresponsive (but still up, just very slow). A fix I
> sent recently seems to really improve that if you're interested in
> testing it (I am :)). You can find it here:
> https://lore.kernel.org/linux-riscv/20240229121056.203419-1-alexghiti@xxxxxxxxxxxx/

I saw the same issue where function_graph was making the system slow on Qemu.
What hardware do you use for testing? or are you testing on Qemu as well?

I tested your patch, it speeds up the process of patching the
instructions so the following
command completes ~2.5 seconds faster compared to without your patch.
$ time echo function_graph > current_tracer

But at runtime the system is still slow and laggy with function_graph,
I guess because my
Qemu setup is not powerful enough to run function_graph.

Thanks,
Puranjay

Next message: Andrew Lunn: "Re: [PATCH net-next v3 03/12] net: ethernet: oa_tc6: implement register read operation"
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In reply to: Alexandre Ghiti: "Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS"
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