Re: [RFC PATCH] arm64: topology: Map PPTT node offset to logic physical package id

[Jeremy Linton]

Hi,

On 06/28/2018 07:12 AM, Sudeep Holla wrote:
> On 28/06/18 12:57, Andrew Jones wrote:
> > On Thu, Jun 28, 2018 at 10:38:24AM +0100, Sudeep Holla wrote:
> > > Hi Shunyong,
> > >
> > > On 28/06/18 10:18, Shunyong Yang wrote:
> > > > As PPTT spec doesn't define the physical package id,
> > > > find_acpi_cpu_topology_package() will return offset of the node with
> > > > Physical package field set when querying physical package id. So, it
> > > > returns 162(0xA2) in following example.
> > > >
> > > > [0A2h 0162   1]                Subtable Type : 00 [Processor Hierarchy
> > > > Node]
> > > > [0A3h 0163   1]                       Length : 1C
> > > > [0A4h 0164   2]                     Reserved : 0000
> > > > [0A6h 0166   4]        Flags (decoded below) : 00000003
> > > >                              Physical package : 1
> > > >                       ACPI Processor ID valid : 1
> > > > [0AAh 0170   4]                       Parent : 00000000
> > > > [0AEh 0174   4]            ACPI Processor ID : 00001000
> > > > [0B2h 0178   4]      Private Resource Number : 00000002
> > > > [0B6h 0182   4]             Private Resource : 0000006C
> > > > [0BAh 0186   4]             Private Resource : 00000084
> > > >
> > > > So, when "cat physical_package" in /sys/devices/system/cpu/cpu0/topology/,
> > > > it will output 162(0xA2). And if some items are added before the node
> > > > above, the output will change to other value.
> > > >
> > > > This patch maps the node offset to a logic package id. It maps the first
> > > > node offset to 0, the second to 1, and so on.
> > > >
> > > > Then, it will not output a big value, such as 162 above. And it will
> > > > not change when some nodes(Physical package not set) are added.
> > > >
> > > > And as long as the nodes with Physical package field set in PPTT keeps
> > > > the real hardware order, the logic id can map to hardware package id to
> > > > some extent.
> > > >
> > > > Hope to get feedback from you.
> > >
> > > Thanks for the patch, but Andrew Jones has also posted a patch[1] which
> > > I had a look but was not sure what is the best approach to fix it yet.
> > > I will think about it and respond to that.
> >
> > I'll send a v1 yet today. The RFC version was actually OK, as the concern
> > with ACPI nodes not being in the expected order wasn't actually a problem.
> > The thread-id or core-id would only be reset to zero when a yet to be
> > remapped core-id (and all its peers) was found when iterating the PEs.
> > Since all peers were handled at the same time, the counter reset was
> > correct, even when the ACPI nodes were out-of-order. The code didn't make
> > that very obvious, though, and there was some room for other cleanups,
> > so I've reworked it. Once I run it through a couple more rounds of testing
> > I'll repost.
>
> OK sure. I liked the approach in Shunyong's patch. I was thinking if we
> can avoid the list and dynamic allocation on each addition and make it
> more simpler.

This one reads simpler, but yes I agree we should try to avoid the 
dynamic allocation.

OTOH, I think that dropping the dynamic allocation leads to an algorithm 
that picks a value and replaces all the matches. Which of course is 
Andrew's patch, although I did have to read it a couple times to get a 
grasp how it works. I'm guessing that is due to the fact that he seems 
to have optimized 3 double loops into a single loop with two individual 
nested loops. AKA its probably more efficient than the naive 
implementation, but readability seems to have suffered a bit in the 
initial version he posted. I'm not sure the optimization is worth it, 
but I'm guessing there is a middle ground which makes it more readable.

Finally, @Shunyong, thanks for putting the effort into this...