On Sep 17, 2015, at 8:17 PM, Catalin Marinas wrote: Hi Catalin,
> On Sun, Sep 13, 2015 at 02:42:17PM +0000, Jungseok Lee wrote: >> Currently, kernel context and interrupts are handled using a single >> kernel stack navigated by sp_el1. This forces many systems to use >> 16KB stack, not 8KB one. Low memory platforms naturally 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 trace struct thread_info using sp_el0 instead >> of sp_el1. This approach helps arm64 align with other architectures >> regarding object_is_on_stack() without additional complexity. > > I'm still trying to understand how this patch works. I initially thought > that we would set SPSel = 0 while in kernel thread mode to make use of > SP_EL0 but I can't find any such code. Do you still use SP_EL1 all the > time and SP_EL0 just for temporary saving the thread stack? Exactly. My first approach was to set SPSel = 0 and implement EL1t Sync and IRQ. This idea originally comes from your comment [1]. A kernel thread could be handled easily and neatly, but it complicated current_thread_info calculation due to a user process. Let's assume that a kernel thread uses SP_EL0 by default. When an interrupt comes in, a core jumps to EL1t IRQ. In case of a user process, a CPU goes into EL1h IRQ when an interrupt raises. To handle this scenario correctly, SPSel or spsr_el1 should be referenced. This reaches to quite big overhead in current_thread_info function. I always keep my mind on simplicity of the function. Thus, I've decided to give up the approach. [1] https://lkml.org/lkml/2015/5/25/454 Best Regards Jungseok Lee -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
