linux-kernel - Re: [PATCH v3] powerpc/pseries: Only wait for dying CPU after call to rtas_stop

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Date:   Wed, 17 Apr 2019 22:00:09 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] powerpc/pseries: Only wait for dying CPU after call to rtas_stop_self()


Hello Nick,

Thank you very much for reviewing this patch!

Nicholas Piggin <npiggin@...il.com> writes:

> Thiago Jung Bauermann's on April 11, 2019 9:08 am:
>>
>> Thiago Jung Bauermann <bauerman@...ux.ibm.com> writes:
>>
>>> 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);
>
> If the CPU state does not go to offline here, you should give up and
> return online, right? Otherwise I think query-cpu-stopped-state can
> get confused by CPUs in idle and you get a false positive.

Can it get confused? My impression from reading the definition for
query-cpu-stopped-state in the PAPR is that it will simply return a
CPU_status value of 2 in that case, meaning that "the processor thread
is not in the RTAS stopped state", but I don't know much about this.

> That race can still happen, we would really need a sequence count check
> over current CPU state to ensure we got a race-free qcss value, but at
> least a check here should make the race implausible to hit.

Actually, since rtas_stop_self() panics if the processor fails to stop
and also since callers of pseries_cpu_die()¹ already assume that it is
going to succeed in stopping the CPU (given that the function returns
void and can't signal an error), a more straightforward way of
eliminating the race is to simply do this:

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 97feb6e79f1a..2331a609f48f 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -215,7 +215,7 @@ static void pseries_cpu_die(unsigned int cpu)
                }
        } else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) {

-               for (tries = 0; tries < 25; tries++) {
+               while (true) {
                        cpu_status = smp_query_cpu_stopped(pcpu);
                        if (cpu_status == QCSS_STOPPED ||
                            cpu_status == QCSS_HARDWARE_ERROR)


What do you think?

--
Thiago Jung Bauermann
IBM Linux Technology Center

¹ dlpar_offline_cpu() and takedown_cpu() in generic code