Re: [Xen-devel] [PATCH 1/8] cpupools: fix state when downing a CPU failed

[Juergen Gross <jgross@xxxxxxxx>]

On 16/07/18 14:19, Jan Beulich wrote:
>>>> On 16.07.18 at 13:47, <jgross@xxxxxxxx> wrote:
>> On 16/07/18 11:17, Jan Beulich wrote:
>>>>>> On 13.07.18 at 11:02, <jgross@xxxxxxxx> wrote:
>>>> On 11/07/18 14:04, Jan Beulich wrote:
>>>>> While I've run into the issue with further patches in place which no
>>>>> longer guarantee the per-CPU area to start out as all zeros, the
>>>>> CPU_DOWN_FAILED processing looks to have the same issue: By not zapping
>>>>> the per-CPU cpupool pointer, cpupool_cpu_add()'s (indirect) invocation
>>>>> of schedule_cpu_switch() will trigger the "c != old_pool" assertion
>>>>> there.
>>>>>
>>>>> Clearing the field during CPU_DOWN_PREPARE is too early (afaict this
>>>>> should not happen before cpu_disable_scheduler()). Clearing it in
>>>>> CPU_DEAD and CPU_DOWN_FAILED would be an option, but would take the same
>>>>> piece of code twice. Since the field's value shouldn't matter while the
>>>>> CPU is offline, simply clear it in CPU_ONLINE and CPU_DOWN_FAILED, but
>>>>> only for other than the suspend/resume case (which gets specially
>>>>> handled in cpupool_cpu_remove()).
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> ---
>>>>> TBD: I think this would better call schedule_cpu_switch(cpu, NULL) from
>>>>>      cpupool_cpu_remove(), but besides that - as per above - likely
>>>>>      being too early, that function has further prereqs to be met. It
>>>>>      also doesn't look as if cpupool_unassign_cpu_helper() could be used
>>>>>      there.
>>>>>
>>>>> --- a/xen/common/cpupool.c
>>>>> +++ b/xen/common/cpupool.c
>>>>> @@ -778,6 +778,8 @@ static int cpu_callback(
>>>>>      {
>>>>>      case CPU_DOWN_FAILED:
>>>>>      case CPU_ONLINE:
>>>>> +        if ( system_state <= SYS_STATE_active )
>>>>> +            per_cpu(cpupool, cpu) = NULL;
>>>>>          rc = cpupool_cpu_add(cpu);
>>>>
>>>> Wouldn't it make more sense to clear the field in cpupool_cpu_add()
>>>> which already is testing system_state?
>>>
>>> Hmm, this may be a matter of taste: I consider the change done here
>>> a prereq to calling the function in the first place. As said in the
>>> description, I actually think this should come earlier, and it's just that
>>> I can't see how to cleanly do so.
> 
> You didn't comment on this one at all, yet it matters for how a v2
> is supposed to look like.

My comment was thought to address this question, too. cpupool_cpu_add()
is handling the special case of resuming explicitly, where the old cpu
assignment to a cpupool is kept. So I believe setting
  per_cpu(cpupool, cpu) = NULL
in the else clause of cpupool_cpu_add() only is better.

>>>> Modifying the condition in cpupool_cpu_add() to
>>>>
>>>>   if ( system_state <= SYS_STATE_active )
>>>>
>>>> at the same time would have the benefit to catch problems in case
>>>> suspending cpus is failing during SYS_STATE_suspend (I'd expect
>>>> triggering the first ASSERT in schedule_cpu_switch() in this case).
>>>
>>> You mean the if() there, not the else? If so - how would the "else"
>>> body then ever be reached? IOW if anything I could only see the
>>> "else" to become "else if ( system_state <= SYS_STATE_active )".
>>
>> Bad wording on my side.
>>
>> I should have written "the condition in cpupool_cpu_add() should match
>> if ( system_state <= SYS_STATE_active )."
>>
>> So: "if ( system_state > SYS_STATE_active )", as the test is for the
>> other case.
> 
> I'd recommend against this, as someone adding a new SYS_STATE_*
> past suspend/resume would quite likely miss this one. The strong
> ordering of states imo should only be used for active and lower states.
> But yes, I could see the if() there to become suspend || resume to
> address the problem you describe.

Yes, this would seem to be a better choice here.

> Coming back to your DOWN_FAILED consideration: Why are you
> thinking this can't happen during suspend? disable_nonboot_cpus()
> uses plain cpu_down() after all.

Right.

DOWN_FAILED is used only once, and that is in cpu_down() after the step
CPU_DOWN_PREPARE returned an error. And CPU_DOWN_PREPARE is only used
for cpufreq driver where it never returns an error, and for cpupools
which don't matter here, as only other components failing at step
CPU_DOWN_PREPARE would lead to calling cpupool/DOWN_FAILED.


Juergen

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