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authorChristoph Lameter <clameter@sgi.com>2008-01-07 23:20:30 -0800
committerChristoph Lameter <christoph@stapp.engr.sgi.com>2008-02-07 17:47:41 -0800
commit1f84260c8ce3b1ce26d4c1d6dedc2f33a3a29c0c (patch)
treed57e305168291f1b0f5f2d42853d2c2419f4140b /Documentation
parent683d0baad3d6e18134927f8c28ee804dbe10fe71 (diff)
SLUB: Alternate fast paths using cmpxchg_local
Provide an alternate implementation of the SLUB fast paths for alloc and free using cmpxchg_local. The cmpxchg_local fast path is selected for arches that have CONFIG_FAST_CMPXCHG_LOCAL set. An arch should only set CONFIG_FAST_CMPXCHG_LOCAL if the cmpxchg_local is faster than an interrupt enable/disable sequence. This is known to be true for both x86 platforms so set FAST_CMPXCHG_LOCAL for both arches. Currently another requirement for the fastpath is that the kernel is compiled without preemption. The restriction will go away with the introduction of a new per cpu allocator and new per cpu operations. The advantages of a cmpxchg_local based fast path are: 1. Potentially lower cycle count (30%-60% faster) 2. There is no need to disable and enable interrupts on the fast path. Currently interrupts have to be disabled and enabled on every slab operation. This is likely avoiding a significant percentage of interrupt off / on sequences in the kernel. 3. The disposal of freed slabs can occur with interrupts enabled. The alternate path is realized using #ifdef's. Several attempts to do the same with macros and inline functions resulted in a mess (in particular due to the strange way that local_interrupt_save() handles its argument and due to the need to define macros/functions that sometimes disable interrupts and sometimes do something else). [clameter: Stripped preempt bits and disabled fastpath if preempt is enabled] Signed-off-by: Christoph Lameter <clameter@sgi.com> Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi> Cc: <linux-arch@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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