| Message ID | 20220913094252.3555240-2-andy.chiu@sifive.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Enable ftrace with kernel preemption for RISC-V | expand |
On Tue, Sep 13, 2022 at 5:44 PM Andy Chiu <andy.chiu@sifive.com> wrote: > > We are introducing a new ftrace mechanism in order to phase out > stop_machine() and enable kernel preemption. The new mechanism requires > ftrace patchable function entries to be 24 bytes and aligned to 4 Byte > boundaries. > > Before applying this patch, the size of the kernel code, with 122465 of > ftrace entries, was at 12.46 MB. Under the same configuration, the size > has increased to 12.99 MB after applying this patch set. > > However, we found the -falign-functions alone was not strong enoungh to > make all functions align as required. In fact, cold functions are not > aligned after turning on optimizations. We consider this is a bug in GCC > and turn off guess-branch-probility as a workaround to align all > functions. Disable pgo static optimization would reduce the code performance. I think we need to fix that problem in GCC, and pgo is a default-enabled optimization. > > GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345 > > Signed-off-by: Andy Chiu <andy.chiu@sifive.com> > Reviewed-by: Greentime Hu <greentime.hu@sifive.com> > --- > arch/riscv/Makefile | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile > index 3fa8ef336822..fd8069f59a59 100644 > --- a/arch/riscv/Makefile > +++ b/arch/riscv/Makefile > @@ -11,7 +11,7 @@ LDFLAGS_vmlinux := > ifeq ($(CONFIG_DYNAMIC_FTRACE),y) > LDFLAGS_vmlinux := --no-relax > KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY > - CC_FLAGS_FTRACE := -fpatchable-function-entry=8 > + CC_FLAGS_FTRACE := -fpatchable-function-entry=12 -falign-functions=4 -fno-guess-branch-probability Another fixup should be covered, eg: +ifeq ($(CONFIG_RISCV_ISA_C),y) + CC_FLAGS_FTRACE := -fpatchable-function-entry=12 +else + CC_FLAGS_FTRACE := -fpatchable-function-entry=6 +endif > endif > > ifeq ($(CONFIG_CMODEL_MEDLOW),y) > -- > 2.36.0 >
On Thu, Sep 15, 2022 at 2:54 PM Guo Ren <guoren@kernel.org> wrote: > > On Tue, Sep 13, 2022 at 5:44 PM Andy Chiu <andy.chiu@sifive.com> wrote: > > > > However, we found the -falign-functions alone was not strong enoungh to > > make all functions align as required. In fact, cold functions are not > > aligned after turning on optimizations. We consider this is a bug in GCC > > and turn off guess-branch-probility as a workaround to align all > > functions. > Disable pgo static optimization would reduce the code performance. I > think we need to fix that problem in GCC, and pgo is a default-enabled > optimization. I would like to see this fix in GCC as well. Or the other way is to reserve extra bytes (like, 2 bytes) and pick an aligned address in software. > > > > GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345 > > > > Signed-off-by: Andy Chiu <andy.chiu@sifive.com> > > Reviewed-by: Greentime Hu <greentime.hu@sifive.com> > > --- > > arch/riscv/Makefile | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile > > index 3fa8ef336822..fd8069f59a59 100644 > > --- a/arch/riscv/Makefile > > +++ b/arch/riscv/Makefile > > @@ -11,7 +11,7 @@ LDFLAGS_vmlinux := > > ifeq ($(CONFIG_DYNAMIC_FTRACE),y) > > LDFLAGS_vmlinux := --no-relax > > KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY > > - CC_FLAGS_FTRACE := -fpatchable-function-entry=8 > > + CC_FLAGS_FTRACE := -fpatchable-function-entry=12 -falign-functions=4 -fno-guess-branch-probability > Another fixup should be covered, eg: > > +ifeq ($(CONFIG_RISCV_ISA_C),y) > + CC_FLAGS_FTRACE := -fpatchable-function-entry=12 > +else > + CC_FLAGS_FTRACE := -fpatchable-function-entry=6 > +endif > Thanks for reminding. Yes, we have to do that. Regards, Andy
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile index 3fa8ef336822..fd8069f59a59 100644 --- a/arch/riscv/Makefile +++ b/arch/riscv/Makefile @@ -11,7 +11,7 @@ LDFLAGS_vmlinux := ifeq ($(CONFIG_DYNAMIC_FTRACE),y) LDFLAGS_vmlinux := --no-relax KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY - CC_FLAGS_FTRACE := -fpatchable-function-entry=8 + CC_FLAGS_FTRACE := -fpatchable-function-entry=12 -falign-functions=4 -fno-guess-branch-probability endif ifeq ($(CONFIG_CMODEL_MEDLOW),y)