dvh...@linux.vnet.ibm.com wrote on 07/22/2010 06:57:18 PM: > Subject > > Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with > CONFIG_PREEMPT on pseries > > On 07/22/2010 03:25 PM, Benjamin Herrenschmidt wrote: > > On Thu, 2010-07-22 at 11:24 -0700, Darren Hart wrote: > >> > >> 1) How can the preempt_count() get mangled across the H_CEDE hcall? > >> 2) Should we call preempt_enable() in cpu_idle() prior to cpu_die() ? > > > > The preempt count is on the thread info at the bottom of the stack. > > > > Can you check the stack pointers ? > > Hi Ben, thanks for looking. > > I instrumented the area around extended_cede_processor() as follows > (please confirm I'm getting the stack pointer correctly). > > while (get_preferred_offline_state(cpu) == CPU_STATE_INACTIVE) { > asm("mr %0,1" : "=r" (sp)); > printk("before H_CEDE current->stack: %lx, pcnt: %x\n", sp, > preempt_count()); > extended_cede_processor(cede_latency_hint); > asm("mr %0,1" : "=r" (sp)); > printk("after H_CEDE current->stack: %lx, pcnt: %x\n", sp, > preempt_count()); > } > > > On Mainline (2.6.33.6, CONFIG_PREEMPT=y) I see this: > Jul 22 18:37:08 igoort1 kernel: before H_CEDE current->stack: > c00000010e9e3ce0, pcnt: 1 > Jul 22 18:37:08 igoort1 kernel: after H_CEDE current->stack: > c00000010e9e3ce0, pcnt: 1 > > This surprised me as preempt_count is 1 before and after, so no > corruption appears to occur on mainline. This makes the pcnt of 65 I see > without the preempt_count()=0 hack very strange. I ran several hundred > off/on cycles. The issue of preempt_count being 1 is still addressed by > this patch however. > > On PREEMPT_RT (2.6.33.5-rt23 - tglx, sorry, rt/2.6.33 next time, promise): > Jul 22 18:51:11 igoort1 kernel: before H_CEDE current->stack: > c000000089bcfcf0, pcnt: 1 > Jul 22 18:51:11 igoort1 kernel: after H_CEDE current->stack: > c000000089bcfcf0, pcnt: ffffffff
I'm not seeing the preempt_count value corrupted with my current set of debug, however, I have added buffers to the thread_info struct, so wonder if I've moved the preempt_count variable out of the way of the corruption. (Still investigating that point..) <Why.. because I had been trying to set a DABR on the preempt_count value to catch the corrupter, and due to hits on the nearby _flags fields, getting false positives..> struct thread_info { |-------struct task_struct *task;|------|-------/* main task structure */ |-------struct exec_domain *exec_domain;|-------/* execution domain */ |-------int|----|-------cpu;|---|-------|-------/* cpu we're on */ |-------int|----|-------pad_buffer[64]; |-------int|----|-------preempt_count;|-|-------/* 0 => preemptable, |-------|-------|-------|-------|-------|------- <0 => BUG */ |-------int|----|-------pad_buffer2[256]; |-------struct restart_block restart_block; |-------unsigned long|--local_flags;|---|-------/* private flags for thread */ |-------/* low level flags - has atomic operations done on it */ |-------unsigned long|--flags ____cacheline_aligned_in_smp; }; > > In both cases the stack pointer appears unchanged. > > Note: there is a BUG triggered in between these statements as the > preempt_count causes the printk to trigger: > Badness at kernel/sched.c:5572 > > Thanks, > > -- > Darren Hart > IBM Linux Technology Center > Real-Time Linux Team _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev