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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> 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 )".

Jan



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