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  1. 18 12月, 2010 6 次提交
    • C
      vmstat: User per cpu atomics to avoid interrupt disable / enable · 7c839120
      由 Christoph Lameter 提交于 
      Currently the operations to increment vm counters must disable interrupts
in order to not mess up their housekeeping of counters.

So use this_cpu_cmpxchg() to avoid the overhead. Since we can no longer
count on preremption being disabled we still have some minor issues.
The fetching of the counter thresholds is racy.
A threshold from another cpu may be applied if we happen to be
rescheduled on another cpu.  However, the following vmstat operation
will then bring the counter again under the threshold limit.

The operations for __xxx_zone_state are not changed since the caller
has taken care of the synchronization needs (and therefore the cycle
count is even less than the optimized version for the irq disable case
provided here).

The optimization using this_cpu_cmpxchg will only be used if the arch
supports efficient this_cpu_ops (must have CONFIG_CMPXCHG_LOCAL set!)

The use of this_cpu_cmpxchg reduces the cycle count for the counter
operations by %80 (inc_zone_page_state goes from 170 cycles to 32).
Signed-off-by: NChristoph Lameter <cl@linux.com>
      7c839120
    • C
      irq_work: Use per cpu atomics instead of regular atomics · 20b87691
      由 Christoph Lameter 提交于 
      The irq work queue is a per cpu object and it is sufficient for
synchronization if per cpu atomics are used. Doing so simplifies
the code and reduces the overhead of the code.

Before:

christoph@linux-2.6$ size kernel/irq_work.o
   text	   data	    bss	    dec	    hex	filename
    451	      8	      1	    460	    1cc	kernel/irq_work.o

After:

christoph@linux-2.6$ size kernel/irq_work.o 
   text	   data	    bss	    dec	    hex	filename
    438	      8	      1	    447	    1bf	kernel/irq_work.o

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NChristoph Lameter <cl@linux.com>
      20b87691
    • T
      Merge branch 'this_cpu_ops' into for-2.6.38 · 05c2d088
      由 Tejun Heo 提交于
      05c2d088
    • C
      cpuops: Use cmpxchg for xchg to avoid lock semantics · 8270137a
      由 Christoph Lameter 提交于 
      Use cmpxchg instead of xchg to realize this_cpu_xchg.

xchg will cause LOCK overhead since LOCK is always implied but cmpxchg
will not.

Baselines:

xchg()		= 18 cycles (no segment prefix, LOCK semantics)
__this_cpu_xchg = 1 cycle

(simulated using this_cpu_read/write, two prefixes. Looks like the
cpu can use loop optimization to get rid of most of the overhead)

Cycles before:

this_cpu_xchg	 = 37 cycles (segment prefix and LOCK (implied by xchg))

After:

this_cpu_xchg	= 11 cycle (using cmpxchg without lock semantics)
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
      8270137a
    • C
      x86: this_cpu_cmpxchg and this_cpu_xchg operations · 7296e08a
      由 Christoph Lameter 提交于 
      Provide support as far as the hardware capabilities of the x86 cpus
allow.

Define CONFIG_CMPXCHG_LOCAL in Kconfig.cpu to allow core code to test for
fast cpuops implementations.

V1->V2:
	- Take out the definition for this_cpu_cmpxchg_8 and move it into
	  a separate patch.

tj: - Reordered ops to better follow this_cpu_* organization.
    - Renamed macro temp variables similar to their existing
      neighbours.
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
      7296e08a
    • C
      percpu: Generic this_cpu_cmpxchg() and this_cpu_xchg support · 2b712442
      由 Christoph Lameter 提交于 
      Generic code to provide new per cpu atomic features

	this_cpu_cmpxchg
	this_cpu_xchg

Fallback occurs to functions using interrupts disable/enable
to ensure correct per cpu atomicity.

Fallback to regular cmpxchg and xchg is not possible since per cpu atomic
semantics include the guarantee that the current cpus per cpu data is
accessed atomically. Use of regular cmpxchg and xchg requires the
determination of the address of the per cpu data before regular cmpxchg
or xchg which therefore cannot be atomically included in an xchg or
cmpxchg without segment override.

tj: - Relocated new ops to conform better to the general organization.
    - This patch contains a trivial comment fix.
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
      2b712442
  3. 17 12月, 2010 34 次提交