Re: [PATCH] powerpc/drmem: cache LMBs in xarray to accelerate lookup

[Fontenot, Nathan]

On 2/3/2020 2:13 PM, Scott Cheloha wrote:
> On Thu, Jan 30, 2020 at 10:09:32AM -0600, Fontenot, Nathan wrote:
>> On 1/29/2020 12:10 PM, Scott Cheloha wrote:
>>> On Tue, Jan 28, 2020 at 05:56:55PM -0600, Nathan Lynch wrote:
>>>> Scott Cheloha <cheloha@xxxxxxxxxxxxx> writes:
>>>>> LMB lookup is currently an O(n) linear search.  This scales poorly when
>>>>> there are many LMBs.
>>>>>
>>>>> If we cache each LMB by both its base address and its DRC index
>>>>> in an xarray we can cut lookups to O(log n), greatly accelerating
>>>>> drmem initialization and memory hotplug.
>>>>>
>>>>> This patch introduces two xarrays of of LMBs and fills them during
>>>>> drmem initialization.  The patch also adds two interfaces for LMB
>>>>> lookup.
>>>>
>>>> Good but can you replace the array of LMBs altogether
>>>> (drmem_info->lmbs)? xarray allows iteration over the members if needed.
>>>
>>> I don't think we can without potentially changing the current behavior.
>>>
>>> The current behavior in dlpar_memory_{add,remove}_by_ic() is to advance
>>> linearly through the array from the LMB with the matching DRC index.
>>>
>>> Iteration through the xarray via xa_for_each_start() will return LMBs
>>> indexed with monotonically increasing DRC indices.> 
>>> Are they equivalent?  Or can we have an LMB with a smaller DRC index
>>> appear at a greater offset in the array?
>>>
>>> If the following condition is possible:
>>>
>>> 	drmem_info->lmbs[i].drc_index > drmem_info->lmbs[j].drc_index
>>>
>>> where i < j, then we have a possible behavior change because
>>> xa_for_each_start() may not return a contiguous array slice.  It might
>>> "leap backwards" in the array.  Or it might skip over a chunk of LMBs.
>>>
>>
>> The LMB array should have each LMB in monotonically increasing DRC Index
>> value. Note that this is set up based on the DT property but I don't recall
>> ever seeing the DT specify LMBs out of order or not being contiguous.
> 
> Is that ordering guaranteed by the PAPR or some other spec or is that
> just a convention?

>From what I remember the PAPR does not specify that DRC indexes are guaranteed
to be contiguous. In past discussions with pHyp developers I had been told that
they always generate contiguous DRC index values but without a specification in
the PAPR that could always break.

-Nathan

> 
> Code like drmem_update_dt_v1() makes me very nervous:
> 
> static int drmem_update_dt_v1(struct device_node *memory,
>                               struct property *prop)
> {
>         struct property *new_prop;
>         struct of_drconf_cell_v1 *dr_cell;
>         struct drmem_lmb *lmb;
>         u32 *p;
> 
>         new_prop = clone_property(prop, prop->length);
>         if (!new_prop)
>                 return -1;
> 
>         p = new_prop->value;
>         *p++ = cpu_to_be32(drmem_info->n_lmbs);
> 
>         dr_cell = (struct of_drconf_cell_v1 *)p;
> 
>         for_each_drmem_lmb(lmb) {
>                 dr_cell->base_addr = cpu_to_be64(lmb->base_addr);
>                 dr_cell->drc_index = cpu_to_be32(lmb->drc_index);
>                 dr_cell->aa_index = cpu_to_be32(lmb->aa_index);
>                 dr_cell->flags = cpu_to_be32(drmem_lmb_flags(lmb));
> 
>                 dr_cell++;
>         }
> 
>         of_update_property(memory, new_prop);
>         return 0;
> }
> 
> If for whatever reason the firmware has a DRC that isn't monotonically
> increasing and we update a firmware property at the wrong offset I have
> no idea what would happen.
> 
> With the array we preserve the order.  Without it we might violate
> some assumption the firmware has made.
>