On Sep 17, 2015, at 10:17 PM, Jungseok Lee wrote: > 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.
This statement is described incorrectly. In case of user process, a CPU goes into EL0 IRQ. Under this context, another interrupt could come in. At this time, a core jumps to EL1h IRQ. My original intention is to describe this situation. Sorry for confusion. 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/
