Re: [PATCH v3 7/7] spapr_drc.c: use DRC reconfiguration to cleanup DIMM unplug state

[David Gibson]

On Fri, Feb 19, 2021 at 05:04:23PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 2/16/21 11:31 PM, David Gibson wrote:
> > On Thu, Feb 11, 2021 at 07:52:46PM -0300, Daniel Henrique Barboza wrote:
> > > Handling errors in memory hotunplug in the pSeries machine is more complex
> > > than any other device type, because there are all the complications that 
> > > other
> > > devices has, and more.
> > > 
> > > For instance, determining a timeout for a DIMM hotunplug must consider if 
> > > it's a
> > > Hash-MMU or a Radix-MMU guest, because Hash guests takes longer to 
> > > hotunplug DIMMs.
> > > The size of the DIMM is also a factor, given that longer DIMMs naturally 
> > > takes
> > > longer to be hotunplugged from the kernel. And there's also the guest 
> > > memory usage to
> > > be considered: if there's a process that is consuming memory that would 
> > > be lost by
> > > the DIMM unplug, the kernel will postpone the unplug process until the 
> > > process
> > > finishes, and then initiate the regular hotunplug process. The first two
> > > considerations are manageable, but the last one is a deal breaker.
> > > 
> > > There is no sane way for the pSeries machine to determine the memory load 
> > > in the guest
> > > when attempting a DIMM hotunplug - and even if there was a way, the guest 
> > > can start
> > > using all the RAM in the middle of the unplug process and invalidate our 
> > > previous
> > > assumptions - and in result we can't even begin to calculate a timeout 
> > > for the
> > > operation. This means that we can't implement a viable timeout mechanism 
> > > for memory
> > > unplug in pSeries.
> > > 
> > > Going back to why we would consider an unplug timeout, the reason is that 
> > > we can't
> > > know if the kernel is giving up the unplug. Turns out that, sometimes, we 
> > > can.
> > > Consider a failed memory hotunplug attempt where the kernel will error 
> > > out with
> > > the following message:
> > > 
> > > 'pseries-hotplug-mem: Memory indexed-count-remove failed, adding any 
> > > removed LMBs'
> > > 
> > > This happens when there is a LMB that the kernel gave up in removing, and 
> > > the LMBs
> > > marked for removal of the same DIMM are now being added back. This 
> > > process happens
> > 
> > We need to be a little careful about terminology here.  From the
> > guest's point of view, there's no such thing as a DIMM, only LMBs.
> > What the guest is doing here is essentially rejecting a single "index
> > + number" DRC unplug request, which corresponds to one DIMM on the
> > qemu side.
> 
> I'll reword this paragraph to avoid using "DIMM" in the context of the guest
> kernel.
> 
> > 
> > > in the pseries kernel in [1], dlpar_memory_remove_by_ic() into 
> > > dlpar_add_lmb(), and
> > > after that update_lmb_associativity_index(). In this function, the kernel 
> > > is configuring
> > > the LMB DRC connector again. Note that this is a valid usage in LOPAR, as 
> > > stated in
> > > section "ibm,configure-connector RTAS Call":
> > > 
> > > 'A subsequent sequence of calls to ibm,configure-connector with the same 
> > > entry from
> > > the “ibm,drc-indexes” or “ibm,drc-info” property will restart the 
> > > configuration of
> > > devices which were not completely configured.'
> > > 
> > > We can use this kernel behavior in our favor. If a DRC connector 
> > > reconfiguration
> > > for a LMB that we marked as unplug pending happens, this indicates that 
> > > the kernel
> > > changed its mind about the unplug and is reasserting that it will keep 
> > > using the
> > > DIMM. In this case, it's safe to assume that the whole DIMM unplug was 
> > > cancelled.
> > > 
> > > This patch hops into rtas_ibm_configure_connector() and, in the scenario 
> > > described
> > > above, clear the unplug state for the DIMM device. This will not solve 
> > > all the
> > > problems we still have with memory unplug, but it will cover this case 
> > > where the
> > > kernel reconfigures LMBs after a failed unplug. We are a bit more 
> > > resilient,
> > > without using an unreliable timeout, and we didn't make the remaining 
> > > error cases
> > > any worse.
> > 
> > I wonder if we could use this as a beginning of a hotplug failure
> > reporting mechanism.  As noted, this is explicitly allowed by PAPR and
> > I think in general it makes sense that a configure-connector would
> > re-assert that the guest is using the resource and we can't unplug it.
> > 
> 
> I think it's worth looking into it. The kernel already does that in case of 
> hotunplug
> failure of LMBs (at least in this particular case), so it's a matter of 
> evaluating
> how hard it is to do the same for e.g. CPUs.
> 
> 
> > Could we extend guests to do an indicative configure-connector on any
> > unplug it knows it can't complete?  Or if configure-connector is too
> > disruptive could we use an (extra) H_SET_INDICATOR to "UNISOLATE"
> > state? If I'm reading right, that should be both permitted and a no-op
> > for existing PAPR implementations, so it should be a pretty safe way
> > to add that indication.
> 
> A quick look in LOPAR shows that set_indicator can be used to report
> hotplug failures (which is a surprise to me, I wasn't aware of it):
> 
> -----
> (Table 13.7, R1-13.5.3.4-4.)
> 
> For all DR options: If this is a DR operation that involves the user insert-
> ing a DR entity, then if the firmware can determine that the inserted entity
> would cause a system disturbance, then the set-indicator RTAS call must
> not unisolate the entity and must return an error status which is unique to 
> the
> particular error.
> -----
> 
> The wording 'would cause a system disturbance' seems generic on purpose, 
> giving
> the firmware/platform the prerrogative to refuse a hotplug for any given
> reason.

Right.  This is about firmware/platform detected errors, which is of
less interest to us at the moment than OS detected errors.

> I also read that there is no rule against using set_indicator with 
> "unisolate" to
> an already unisolated resource. It would be a no-op.
> 
> I don't think we would find fierce opposition if we propose an addendum such 
> as:
> 
> "For all DR options: If this is a DR operation that involves the user removing
> ing a DR entity, then if the partition operational system can determine that
> removing the entity would cause a system disturbance, then the set-indicator 
> RTAS
> call can be used with the 'unisolate' value to inform the platform that the 
> entity
> can not be removed at that time."
> 
> This would be enough to accomplish what we're aiming for using set_indicator 
> and
> unisolate. Then we have 2 options to signal a failed unplug - 
> configure-connector
> and unisolating the device. The guest can use whichever is easier or makes 
> more
> sense.

Sounds good, let's do it.  Because it's a no-op currently, I also
think we can go ahead with an implementation even without waiting for
a PAPR change to be approved.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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