Re: [PATCH 0/2] pseries/hotplug: Change the default behaviour of cede_offline

Naveen N. Rao
2019-09-18T06:53:03Z

Michael Ellerman wrote:
> "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> writes:
>> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
>>
>> Currently on Pseries Linux Guests, the offlined CPU can be put to one
>> of the following two states:
>> - Long term processor cede (also called extended cede)
>> - Returned to the Hypervisor via RTAS "stop-self" call.
>>
>> This is controlled by the kernel boot parameter "cede_offline=on/off".
>>
>> By default the offlined CPUs enter extended cede.
>
> Since commit 3aa565f53c39 ("powerpc/pseries: Add hooks to put the CPU into an appropriate offline state") (Nov 2009)
>
> Which you wrote :)
>
> Why was that wrong?
>
>> The PHYP hypervisor considers CPUs in extended cede to be "active"
>> since the CPUs are still under the control fo the Linux Guests. Hence, when we change the
>> SMT modes by offlining the secondary CPUs, the PURR and the RWMR SPRs
>> will continue to count the values for offlined CPUs in extended cede
>> as if they are online.
>>
>> One of the expectations with PURR is that the for an interval of time,
>> the sum of the PURR increments across the online CPUs of a core should
>> equal the number of timebase ticks for that interval.
>>
>> This is currently not the case.
>
> But why does that matter? It's just some accounting stuff, does it
> actually break something meaningful?

Yes, this broke lparstat at the very least (though its quite unfortunate
we took so long to notice).

With SMT disabled, and under load:
$ sudo lparstat 1 10

System Configuration
type=Shared mode=Uncapped smt=Off lcpu=2 mem=7759616 kB cpus=6 ent=1.00

%user %sys %wait %idle physc %entc lbusy vcsw phint
----- ----- ----- ----- ----- ----- ----- ----- -----
100.00 0.00 0.00 0.00 1.10 110.00 100.00 128784460 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128784860 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128785260 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128785662 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128786062 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128786462 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128786862 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128787262 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128787664 0
100.00 0.00 0.00 0.00 1.07 107.00 100.00 128788064 0

With cede_offline=off:
$ sudo lparstat 1 10

System Configuration
type=Shared mode=Uncapped smt=Off lcpu=2 mem=7759616 kB cpus=6 ent=1.00

%user %sys %wait %idle physc %entc lbusy vcsw phint
----- ----- ----- ----- ----- ----- ----- ----- -----
100.00 0.00 0.00 0.00 1.94 194.00 100.00 128961588 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128961988 0
100.00 0.00 0.00 0.00 inf inf 100.00 128962392 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128962792 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128963192 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128963592 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128963992 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128964392 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128964792 0
100.00 0.00 0.00 0.00 1.91 191.00 100.00 128965194 0

[The 'inf' values there show a different bug]

Also, since we expose [S]PURR through sysfs, any tools that make use of
that directly are also affected due to this.

- Naveen