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sparc64: Improvde documentation and readability of atomic backoff code.

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Document what's going on in asm/backoff.h with a large and descriptive
comment.  Refer to it above the cpu_relax() definition in
asm/processor_64.h

Rename the pause patching section to have "3insn" in it's name like
the other patching sections do.

Based upon feedback from Sam Ravnborg.

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2012-10-28 13:04:47 -07:00
parent e9b9eb59ff
commit 187818cd6a
5 changed files with 55 additions and 10 deletions
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42
arch/sparc/include/asm/backoff.h
View file

@ -1,6 +1,46 @@
#ifndef _SPARC64_BACKOFF_H
#define _SPARC64_BACKOFF_H
/* The macros in this file implement an exponential backoff facility
* for atomic operations.
*
* When multiple threads compete on an atomic operation, it is
* possible for one thread to be continually denied a successful
* completion of the compare-and-swap instruction. Heavily
* threaded cpu implementations like Niagara can compound this
* problem even further.
*
* When an atomic operation fails and needs to be retried, we spin a
* certain number of times. At each subsequent failure of the same
* operation we double the spin count, realizing an exponential
* backoff.
*
* When we spin, we try to use an operation that will cause the
* current cpu strand to block, and therefore make the core fully
* available to any other other runnable strands. There are two
* options, based upon cpu capabilities.
*
* On all cpus prior to SPARC-T4 we do three dummy reads of the
* condition code register. Each read blocks the strand for something
* between 40 and 50 cpu cycles.
*
* For SPARC-T4 and later we have a special "pause" instruction
* available. This is implemented using writes to register %asr27.
* The cpu will block the number of cycles written into the register,
* unless a disrupting trap happens first. SPARC-T4 specifically
* implements pause with a granularity of 8 cycles. Each strand has
* an internal pause counter which decrements every 8 cycles. So the
* chip shifts the %asr27 value down by 3 bits, and writes the result
* into the pause counter. If a value smaller than 8 is written, the
* chip blocks for 1 cycle.
*
* To achieve the same amount of backoff as the three %ccr reads give
* on earlier chips, we shift the backoff value up by 7 bits. (Three
* %ccr reads block for about 128 cycles, 1 << 7 == 128) We write the
* whole amount we want to block into the pause register, rather than
* loop writing 128 each time.
*/
#define BACKOFF_LIMIT (4 * 1024)
#ifdef CONFIG_SMP
@ -16,7 +56,7 @@
88: rd %ccr, %g0; \
rd %ccr, %g0; \
rd %ccr, %g0; \
.section .pause_patch,"ax"; \
.section .pause_3insn_patch,"ax";\
.word 88b; \
sllx tmp, 7, tmp; \
wr tmp, 0, %asr27; \