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Subject[RFC 00/15] x86_64: Optimize percpu accesses

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.

Based on linux-2.6.tip/master with following patches applied:

[PATCH 1/1] x86: Add check for node passed to node_to_cpumask V3
[PATCH 1/1] x86: Change _node_to_cpumask_ptr to return const ptr
[PATCH 1/1] sched: Reduce stack size in isolated_cpu_setup()
[PATCH 1/1] kthread: Reduce stack pressure in create_kthread and kthreadd

Signed-off-by: Christoph Lameter <>
Signed-off-by: Mike Travis <>


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