On Mon, Feb 23, 2026 at 03:18:17PM +0000, Puranjay Mohan wrote: > On Sat, Feb 21, 2026 at 12:15 PM Conor Dooley <[email protected]> wrote: > > > > Hey, > > > > On Tue, Apr 08, 2025 at 02:08:34AM +0800, Andy Chiu wrote: > > > From: Puranjay Mohan <[email protected]> > > > > > > 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. > > > > > > We've benchamrked this with the ftrace_ops sample module on Spacemit K1 > > > Jupiter: > > > > > > Without this patch: > > > > > > baseline (Linux rivos 6.14.0-09584-g7d06015d936c #3 SMP Sat Mar 29 > > > +-----------------------+-----------------+----------------------------+ > > > | Number of tracers | Total time (ns) | Per-call average time | > > > |-----------------------+-----------------+----------------------------| > > > | Relevant | Irrelevant | 100000 calls | Total (ns) | Overhead (ns) | > > > |----------+------------+-----------------+------------+---------------| > > > | 0 | 0 | 1357958 | 13 | - | > > > | 0 | 1 | 1302375 | 13 | - | > > > | 0 | 2 | 1302375 | 13 | - | > > > | 0 | 10 | 1379084 | 13 | - | > > > | 0 | 100 | 1302458 | 13 | - | > > > | 0 | 200 | 1302333 | 13 | - | > > > |----------+------------+-----------------+------------+---------------| > > > | 1 | 0 | 13677833 | 136 | 123 | > > > | 1 | 1 | 18500916 | 185 | 172 | > > > | 1 | 2 | 22856459 | 228 | 215 | > > > | 1 | 10 | 58824709 | 588 | 575 | > > > | 1 | 100 | 505141584 | 5051 | 5038 | > > > | 1 | 200 | 1580473126 | 15804 | 15791 | > > > |----------+------------+-----------------+------------+---------------| > > > | 1 | 0 | 13561000 | 135 | 122 | > > > | 2 | 0 | 19707292 | 197 | 184 | > > > | 10 | 0 | 67774750 | 677 | 664 | > > > | 100 | 0 | 714123125 | 7141 | 7128 | > > > | 200 | 0 | 1918065668 | 19180 | 19167 | > > > +----------+------------+-----------------+------------+---------------+ > > > > > > Note: per-call overhead is estimated relative to the baseline case with > > > 0 relevant tracers and 0 irrelevant tracers. > > > > > > With this patch: > > > > > > v4-rc4 (Linux rivos 6.14.0-09598-gd75747611c93 #4 SMP Sat Mar 29 > > > +-----------------------+-----------------+----------------------------+ > > > | Number of tracers | Total time (ns) | Per-call average time | > > > |-----------------------+-----------------+----------------------------| > > > | Relevant | Irrelevant | 100000 calls | Total (ns) | Overhead (ns) | > > > |----------+------------+-----------------+------------+---------------| > > > | 0 | 0 | 1459917 | 14 | - | > > > | 0 | 1 | 1408000 | 14 | - | > > > | 0 | 2 | 1383792 | 13 | - | > > > | 0 | 10 | 1430709 | 14 | - | > > > | 0 | 100 | 1383791 | 13 | - | > > > | 0 | 200 | 1383750 | 13 | - | > > > |----------+------------+-----------------+------------+---------------| > > > | 1 | 0 | 5238041 | 52 | 38 | > > > | 1 | 1 | 5228542 | 52 | 38 | > > > | 1 | 2 | 5325917 | 53 | 40 | > > > | 1 | 10 | 5299667 | 52 | 38 | > > > | 1 | 100 | 5245250 | 52 | 39 | > > > | 1 | 200 | 5238459 | 52 | 39 | > > > |----------+------------+-----------------+------------+---------------| > > > | 1 | 0 | 5239083 | 52 | 38 | > > > | 2 | 0 | 19449417 | 194 | 181 | > > > | 10 | 0 | 67718584 | 677 | 663 | > > > | 100 | 0 | 709840708 | 7098 | 7085 | > > > | 200 | 0 | 2203580626 | 22035 | 22022 | > > > +----------+------------+-----------------+------------+---------------+ > > > > > > 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 122ns to 38ns). This is largely > > > due to permitting calls to dynamically-allocated ftrace_ops without > > > going through ftrace_ops_list_func. > > > > > > Signed-off-by: Puranjay Mohan <[email protected]> > > > > > > [update kconfig, asm, refactor] > > > > > > Signed-off-by: Andy Chiu <[email protected]> > > > Tested-by: Björn Töpel <[email protected]> > > > > I bisected a boot failure to this commit [c217157bcd1df ("riscv: > > Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS")] yesterday, that appears > > to be affecting all LLVM versions that I currently have installed. From > > some initial testing of Kconfig options, it looks like the issue is > > CFI_CLANG related because when I disable CFI_CLANG things work once > > more. Since this option depends on !CFI_CLANG, but is def_bool y, I > > modified Kconfig to force disable it at all times and tested > > !DYNAMIC_FTRACE_WITH_CALL_OPS && !CFG_CLANG, which did boot. > > > > I dunno anything about what's going on in this patch, but so little in > > it relates to having DYNAMIC_FTRACE_WITH_CALL_OPS, that I was able to > > figure out that the problem is -fpatchable-function-entry=8,4 > > > > DYNAMIC_FTRACE_WITH_CALL_OPS can't work together with CFI_CLANG. > > arm64 has: > > select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS \ > if (DYNAMIC_FTRACE_WITH_ARGS && !CFI && \ > (CC_IS_CLANG || !CC_OPTIMIZE_FOR_SIZE)) > > would need something similar for riscv if not already done.
I think you've misunderstood my email. We already have:
select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if (DYNAMIC_FTRACE_WITH_ARGS
&& !CFI)
The problem is that the patch broke using CFI_CLANG, due to the
fpatchable-function-entry change.
Cheers,
Conor.
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