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

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Message-Id: <1555552271.zy2w4ddjqe.astroid@bobo.none>
Date:   Thu, 18 Apr 2019 11:55:06 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.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()

Thiago Jung Bauermann's on April 18, 2019 11:00 am:
> 
> 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.

In QEMU (non-KVM) mode, qcss I think may get confused between H_CEDE
and rtas-stop-self. KVM mode may be okay because H_CEDE is handled in
the kernel.

>> 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?

Yeah I think that may be a good idea, just makes things much simpler.

Thanks,
Nick