Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

From: Andy Chiu
Date: Fri Mar 08 2024 - 03:49:16 EST

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On Thu, Mar 7, 2024 at 8:19 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
>
> 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.

IIRC it should be fine if you're building with Clang. The
"-falign-function" flag for gcc is not aligning cold functions. So I
think we'd need some Kconfig checks for the difference.

>
> >
> > [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.

Yes, it's atomic. But it doesn't guarantee ordering if fence.i is not
executed, IIUC. I am not sure if CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
assumes seeing the updated ftrace_rec argument right after the patch
completes. If that is the case then probably a batched fence.i is
needed on local + remote cores.

>
> > > +}
> > > +
> > > +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
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@xxxxxxxxxxxxxxxxxxx
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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