Messages in this thread | | | Subject | Re: [PATCH v3] arm64: Introduce IRQ stack | From | AKASHI Takahiro <> | Date | Mon, 5 Oct 2015 15:37:11 +0900 |
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On 10/04/2015 11:32 PM, Jungseok Lee wrote: > On Oct 3, 2015, at 1:23 AM, James Morse wrote: > >> Hi, > > Hi James, > >> >> On 22/09/15 13:11, Jungseok Lee wrote: >>> Currently, kernel context and interrupts are handled using a single >>> kernel stack navigated by sp_el1. This forces a system to use 16KB >>> stack, not 8KB one. This restriction makes low memory platforms suffer >>> from memory pressure accompanied by performance degradation. >>> >>> This patch addresses the issue as introducing a separate percpu IRQ >>> stack to handle both hard and soft interrupts with two ground rules: >>> >>> - Utilize sp_el0 in EL1 context, which is not used currently >>> - Do not complicate current_thread_info calculation >>> >>> It is a core concept to directly retrieve struct thread_info from >>> sp_el0. This approach helps to prevent text section size from being >>> increased largely as removing masking operation using THREAD_SIZE >>> in tons of places. >> >> One observed change in behaviour: >> Any stack-unwinding now stops at el1_irq(), which is the bottom of the irq >> stack. This shows up with perf (using incantation [0]), and with any calls >> to dump_stack() (which actually stops the frame before el1_irq()). >> >> I don't know if this will break something, (perf still seems to work) - but >> it makes the panic() output less useful, as all the 'other' cpus print: > > Agreed. A process stack should be walked to deliver useful information. > > There are two approaches I've tried as experimental. > > 1) Link IRQ stack to a process one via frame pointer > As saving x29 and elr_el1 into IRQ stack and then updating x29, IRQ stack > could be linked to a process one. It is similar to your patch except some > points. However, it might complicate "stack tracer on ftrace" issue.
Well, as far as object_is_on_stack() works correctly, stack tracer will not follow an interrupt stack even if unwind_frame() might traverse from an interrupt stack to a process stack. See check_stack().
Under this assumption, I'm going to simplify my "stack tracer" bugfix by removing interrupt-related nasty hacks that I described in RFC.
Thanks, -Takahiro AKASHI
> 2) Walk a process stack followed by IRQ one > This idea, which is straightforward, comes from x86 implementation [1]. The > approach might be orthogonal to "stack tracer on ftrace" issue. In this case, > unfortunately, a top bit comparison of stack pointer cannot be adopted due to > a necessity of a final snapshot of a process stack pointer, which is struct > irq_stack::thread_sp in v2 patch. > > Which one is your favorite? or any ideas? > > BTW, I have another question. Is it reasonable to introduce THREAD_SIZE as a > kernel configuration option like page size for the sake of convenience because > a combination of ARM64 and a small ram is not unusual in real practice? Needless > to say, a patch, reducing the size, can be managed as out of mainline tree one. > > [1] arch/x86/kernel/dumpstack_64.c > > Best Regards > Jungseok Lee >
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