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

[Naveen N. Rao]

Michael Ellerman wrote:
> "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx> writes:
> > From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>
> >
> > 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