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SubjectRe: [RFC 00/15] x86_64: Optimize percpu accesses
Mike Travis wrote:
> This patchset provides the following:
> * Cleanup: Fix early references to cpumask_of_cpu(0)
> Provides an early cpumask_of_cpu(0) usable before the cpumask_of_cpu_map
> is allocated and initialized.
> * Generic: Percpu infrastructure to rebase the per cpu area to zero
> This provides for the capability of accessing the percpu variables
> using a local register instead of having to go through a table
> on node 0 to find the cpu-specific offsets. It also would allow
> atomic operations on percpu variables to reduce required locking.
> Uses a new config var HAVE_ZERO_BASED_PER_CPU to indicate to the
> generic code that the arch has this new basing.
> (Note: split into two patches, one to rebase percpu variables at 0,
> and the second to actually use %gs as the base for percpu variables.)
> * x86_64: Fold pda into per cpu area
> Declare the pda as a per cpu variable. This will move the pda
> area to an address accessible by the x86_64 per cpu macros.
> Subtraction of __per_cpu_start will make the offset based from
> the beginning of the per cpu area. Since %gs is pointing to the
> pda, it will then also point to the per cpu variables and can be
> accessed thusly:
> %gs:[&per_cpu_xxxx - __per_cpu_start]
> * x86_64: Rebase per cpu variables to zero
> Take advantage of the zero-based per cpu area provided above.
> Then we can directly use the x86_32 percpu operations. x86_32
> offsets %fs by __per_cpu_start. x86_64 has %gs pointing directly
> to the pda and the per cpu area thereby allowing access to the
> pda with the x86_64 pda operations and access to the per cpu
> variables using x86_32 percpu operations.

The bulk of this series is pda_X to x86_X_percpu conversion. This seems
like pointless churn to me; there's nothing inherently wrong with the
pda_X interfaces, and doing this transformation doesn't get us any
closer to unifying 32 and 64 bit.

I think we should start devolving things out of the pda in the other
direction: make a series where each patch takes a member of struct
x8664_pda, converts it to a per-cpu variable (where possible, the same
one that 32-bit uses), and updates all the references accordingly. When
the pda is as empty as it can be, we can look at removing the
pda-specific interfaces.


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