Hi Darren, On Thu, Jul 22, 2010 at 11:24:13AM -0700, Darren Hart wrote: > > With some instrumentation we were able to determine that the > preempt_count() appears to change across the extended_cede_processor() > call. Specifically across the plpar_hcall_norets(H_CEDE) call. On > PREEMPT_RT we call this with preempt_count=1 and return with > preempt_count=0xffffffff. On mainline with CONFIG_PREEMPT=y, the value > is different (0x65) but is still incorrect.
I was trying to reproduce this issue on a 2.6.33.7-rt29 kernel. I could easily reproduce this on the RT kernel and not the non-RT kernel. However, I hit it every single time I did a cpu online operation. I also noticed that the issue persists even when I disable H_CEDE by passing the "cede_offline=0" kernel commandline parameter. Could you pl confirm if you observe the same in your setup ? However, the issue still remains. Will spend few cycles looking into this issue. > > Also of interest is that this path > cpu_idle()->cpu_die()->pseries_mach_cpu_die() to start_secondary() > enters with a preempt_count=1 if it wasn't corrupted across the hcall. > The early boot path from _start however appears to call > start_secondary() with a preempt_count of 0. > > The following patch is most certainly not correct, but it does eliminate > the situation on mainline 100% of the time (there is still a 25% > reproduction rate on PREEMPT_RT). Can someone comment on: > > 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() ? > > Hacked-up-by: Darren Hart <dvh...@us.ibm.com> > > Index: linux-2.6.33.6/arch/powerpc/platforms/pseries/hotplug-cpu.c > =================================================================== > --- linux-2.6.33.6.orig/arch/powerpc/platforms/pseries/hotplug-cpu.c > +++ linux-2.6.33.6/arch/powerpc/platforms/pseries/hotplug-cpu.c > @@ -138,6 +138,7 @@ static void pseries_mach_cpu_die(void) > * Kernel stack will be reset and start_secondary() > * will be called to continue the online operation. > */ > + preempt_count() = 0; > start_secondary_resume(); > } > } > > -- Regards, Ankita Garg (ank...@in.ibm.com) Linux Technology Center IBM India Systems & Technology Labs, Bangalore, India _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
