Hello,
Ping? -- Thiago Jung Bauermann IBM Linux Technology Center Thiago Jung Bauermann <bauer...@linux.ibm.com> writes: > When testing DLPAR CPU add/remove on a system under stress, > pseries_cpu_die() doesn't wait long enough for a CPU to die: > > [ 446.983944] cpu 148 (hwid 148) Ready to die... > [ 446.984062] cpu 149 (hwid 149) Ready to die... > [ 446.993518] cpu 150 (hwid 150) Ready to die... > [ 446.993543] Querying DEAD? cpu 150 (150) shows 2 > [ 446.994098] cpu 151 (hwid 151) Ready to die... > [ 447.133726] cpu 136 (hwid 136) Ready to die... > [ 447.403532] cpu 137 (hwid 137) Ready to die... > [ 447.403772] cpu 138 (hwid 138) Ready to die... > [ 447.403839] cpu 139 (hwid 139) Ready to die... > [ 447.403887] cpu 140 (hwid 140) Ready to die... > [ 447.403937] cpu 141 (hwid 141) Ready to die... > [ 447.403979] cpu 142 (hwid 142) Ready to die... > [ 447.404038] cpu 143 (hwid 143) Ready to die... > [ 447.513546] cpu 128 (hwid 128) Ready to die... > [ 447.693533] cpu 129 (hwid 129) Ready to die... > [ 447.693999] cpu 130 (hwid 130) Ready to die... > [ 447.703530] cpu 131 (hwid 131) Ready to die... > [ 447.704087] Querying DEAD? cpu 132 (132) shows 2 > [ 447.704102] cpu 132 (hwid 132) Ready to die... > [ 447.713534] cpu 133 (hwid 133) Ready to die... > [ 447.714064] Querying DEAD? cpu 134 (134) shows 2 > > This is a race between one CPU stopping and another one calling > pseries_cpu_die() to wait for it to stop. That function does a short busy > loop calling RTAS query-cpu-stopped-state on the stopping CPU to verify > that it is stopped, but I think there's a lot for the stopping CPU to do > which may take longer than this loop allows. > > As can be seen in the dmesg right before or after the "Querying DEAD?" > messages, if pseries_cpu_die() waited a little longer it would have seen > the CPU in the stopped state. > > What I think is going on is that CPU 134 was inactive at the time it was > unplugged. In that case, dlpar_offline_cpu() calls H_PROD on that CPU and > immediately calls pseries_cpu_die(). Meanwhile, the prodded CPU activates > and start the process of stopping itself. The busy loop is not long enough > to allow for the CPU to wake up and complete the stopping process. > > This can be a problem because if the busy loop finishes too early, then the > kernel may offline another CPU before the previous one finished dying, > which would lead to two concurrent calls to rtas-stop-self, which is > prohibited by the PAPR. > > We can make the race a lot more even if we only start querying if the CPU > is stopped when the stopping CPU is close to call rtas_stop_self(). Since > pseries_mach_cpu_die() sets the CPU current state to offline almost > immediately before calling rtas_stop_self(), we use that as a signal that > it is either already stopped or very close to that point, and we can start > the busy loop. > > As suggested by Michael Ellerman, this patch also changes the busy loop to > wait for a fixed amount of wall time. Based on the measurements that > Gautham did on a POWER9 system, in successful cases of > smp_query_cpu_stopped(cpu) returning affirmative, the maximum time spent > inside the loop was was 10 ms. This patch loops for 20 ms just be sure. > > Signed-off-by: Thiago Jung Bauermann <bauer...@linux.ibm.com> > Analyzed-by: Gautham R Shenoy <e...@linux.vnet.ibm.com> > --- > arch/powerpc/platforms/pseries/hotplug-cpu.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > I have seen this problem since v4.8. Should this patch go to stable as > well? > > Changes since v2: > - Increaded busy loop to 200 iterations so that it can last up to 20 ms > (suggested by Gautham). > - Changed commit message to include Gautham's remarks. > > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c > b/arch/powerpc/platforms/pseries/hotplug-cpu.c > index 97feb6e79f1a..ac6dc35ab829 100644 > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c > @@ -214,13 +214,22 @@ static void pseries_cpu_die(unsigned int cpu) > msleep(1); > } > } else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) { > + /* > + * If the current state is not offline yet, it means that the > + * dying CPU (which is either in pseries_mach_cpu_die() or in > + * the process of getting there) didn't have a chance yet to > + * call rtas_stop_self() and therefore it's too early to query > + * if the CPU is stopped. > + */ > + spin_event_timeout(get_cpu_current_state(cpu) == > CPU_STATE_OFFLINE, > + 100000, 100); > > - for (tries = 0; tries < 25; tries++) { > + for (tries = 0; tries < 200; tries++) { > cpu_status = smp_query_cpu_stopped(pcpu); > if (cpu_status == QCSS_STOPPED || > cpu_status == QCSS_HARDWARE_ERROR) > break; > - cpu_relax(); > + udelay(100); > } > } >