On Wed, Oct 21, 2020 at 01:45:42PM +0100, Russell King - ARM Linux admin wrote: > On Wed, Oct 21, 2020 at 02:42:48PM +0200, Arnd Bergmann wrote: > > (also resending this reply from @kernel.org) > > > > On Fri, Oct 16, 2020 at 12:19 PM Arnd Bergmann <a...@arndb.de> wrote: > > > On Thu, Oct 8, 2020 at 9:20 AM Maninder Singh <maninder...@samsung.com> > > > wrote: > > > > > > > > This patch adds code for switching to IRQ stack. > > > > IRQ stack and Kernel SVC stack have below design. > > > > > > > > IRQ STACK:- > > > > ------------ IRQ stack top > > > > | | > > > > ------------ > > > > . . > > > > . . > > > > . . > > > > ------------ > > > > | sp | <- irq_stack_base + 0x8 > > > > ------------ > > > > | fp | <- irq_stack_base + 0x4 > > > > ------------ > > > > |tinfo_ptr | /* pointer to thread info */ > > > > irq_stack_ptr --> ------------ IRQ stack base > > > > > > > > Kernel SVC stack:- > > > > ------------ Kernel stack top > > > > | | > > > > ------------ > > > > . . > > > > . . > > > > . . > > > > ------------ > > > > | | > > > > | | > > > > ------------ > > > > |tinfo_ptr | /* pointer to thread info */ > > > > ------------ Kernel stack base > > > > > > The extra indirection doesn't look great, and I don't see any of the > > > other architectures need that. Since we can access percpu data > > > without going through thread_info, maybe doing the same as > > > x86 would work here: > > > > > > - define 'current' as 'this_cpu_read_stable(current_task);' > > > - convert to CONFIG_THREAD_INFO_IN_TASK > > That means we need to also code that up in assembly - remember, we > need to access thread_info from assembly code.
Note also that there is a circular dependency involved. If you make thread_info accessible via per-cpu, then: #ifndef __my_cpu_offset #define __my_cpu_offset per_cpu_offset(raw_smp_processor_id()) #endif #ifdef CONFIG_DEBUG_PREEMPT #define my_cpu_offset per_cpu_offset(smp_processor_id()) #else #define my_cpu_offset __my_cpu_offset #endif smp_processor_id() ultimately ends up as raw_smp_processor_id() which is: #define raw_smp_processor_id() (current_thread_info()->cpu) and if current_thread_info() itself involves reading from per-cpu data, we end up recursing... infinitely. This is why I said in the other thread: "We don't do it because we don't have a separate register to be able to store the thread_info pointer, and copying that lump between the SVC and IRQ stack will add massively to IRQ latency, especially for older machines." -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
