On Tue, Sep 14, 2021 at 02:10:28PM +0200, Ard Biesheuvel wrote: > Commit c65eacbe290b ("sched/core: Allow putting thread_info into > task_struct") mentions that, along with moving thread_info into > task_struct, the cpu field is moved out of the former into the latter, > but does not explain why.
>From what I recall of talking to Andy around that time, when converting arm64 over, the theory was that over time we'd move more and more out of thread_info and into task_struct or thread_struct, until task_struct supplanted thread_info entirely, and that all became generic. I think the key gain there was making things more *generic*, and there are other ways we could do that in future without moving more into task_struct (e.g. with a geenric thread_info and arch_thread_info inside that). With that in mind, and given the diffstat, I think this is worthwhile. FWIW, for the series: Acked-by: Mark Rutland <mark.rutl...@arm.com> Mark. > While collaborating with Keith on adding THREAD_INFO_IN_TASK support to > ARM, we noticed that keeping CPU in task_struct is problematic for > architectures that define raw_smp_processor_id() in terms of this field, > as it requires linux/sched.h to be included, which causes a lot of pain > in terms of circular dependencies (or 'header soup', as the original > commit refers to it). > > For examples of how existing architectures work around this, please > refer to patches #6 or #7. In the former case, it uses an awful > asm-offsets hack to index thread_info/current without using its type > definition. The latter approach simply keeps a copy of the task_struct > CPU field in thread_info, and keeps it in sync at context switch time. > > Patch #8 reverts this latter approach for ARM, but this code is still > under review so it does not currently apply to mainline. > > We also discussed introducing yet another Kconfig symbol to indicate > that the arch has THREAD_INFO_IN_TASK enabled but still prefers to keep > its CPU field in thread_info, but simply keeping it in thread_info in > all cases seems to be the cleanest approach here. > > Cc: Keith Packard <keith...@amazon.com> > Cc: Russell King <li...@armlinux.org.uk> > Cc: Catalin Marinas <catalin.mari...@arm.com> > Cc: Will Deacon <w...@kernel.org> > Cc: Michael Ellerman <m...@ellerman.id.au> > Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> > Cc: Christophe Leroy <christophe.le...@csgroup.eu> > Cc: Paul Mackerras <pau...@samba.org> > Cc: Paul Walmsley <paul.walms...@sifive.com> > Cc: Palmer Dabbelt <pal...@dabbelt.com> > Cc: Albert Ou <a...@eecs.berkeley.edu> > Cc: Heiko Carstens <h...@linux.ibm.com> > Cc: Vasily Gorbik <g...@linux.ibm.com> > Cc: Christian Borntraeger <borntrae...@de.ibm.com> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Ingo Molnar <mi...@redhat.com> > Cc: Borislav Petkov <b...@alien8.de> > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Kees Cook <keesc...@chromium.org> > Cc: Andy Lutomirski <l...@kernel.org> > Cc: Linus Torvalds <torva...@linux-foundation.org> > Cc: Arnd Bergmann <a...@arndb.de> > Cc: linux-arm-ker...@lists.infradead.org > Cc: linuxppc-dev@lists.ozlabs.org > Cc: linux-ri...@lists.infradead.org > Cc: linux-s...@vger.kernel.org > > Ard Biesheuvel (8): > arm64: add CPU field to struct thread_info > x86: add CPU field to struct thread_info > s390: add CPU field to struct thread_info > powerpc: add CPU field to struct thread_info > sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y > powerpc: smp: remove hack to obtain offset of task_struct::cpu > riscv: rely on core code to keep thread_info::cpu updated > ARM: rely on core code to keep thread_info::cpu updated > > arch/arm/include/asm/switch_to.h | 14 -------------- > arch/arm/kernel/smp.c | 3 --- > arch/arm64/include/asm/thread_info.h | 1 + > arch/arm64/kernel/asm-offsets.c | 2 +- > arch/arm64/kernel/head.S | 2 +- > arch/powerpc/Makefile | 11 ----------- > arch/powerpc/include/asm/smp.h | 17 +---------------- > arch/powerpc/include/asm/thread_info.h | 3 +++ > arch/powerpc/kernel/asm-offsets.c | 4 +--- > arch/powerpc/kernel/smp.c | 2 +- > arch/riscv/kernel/asm-offsets.c | 1 - > arch/riscv/kernel/entry.S | 5 ----- > arch/riscv/kernel/head.S | 1 - > arch/s390/include/asm/thread_info.h | 1 + > arch/x86/include/asm/thread_info.h | 3 +++ > include/linux/sched.h | 6 +----- > kernel/sched/sched.h | 4 ---- > 17 files changed, 14 insertions(+), 66 deletions(-) > > -- > 2.30.2 >