For atomic backoff, we just loop over an exponentially backed off
counter. This is extremely ineffective as it doesn't actually yield
the cpu strand so that other competing strands can use the cpu core.
In cpus previous to SPARC-T4 we have to do this in a slightly hackish
way, by doing an operation with no side effects that also happens to
mark the strand as unavailable.
The mechanism we choose for this is three reads of the %ccr
(condition-code) register into %g0 (the zero register).
SPARC-T4 has an explicit "pause" instruction, and we'll make use of
that in a subsequent commit.
Yield strands also in cpu_relax(). We really should have done this a
very long time ago.
Signed-off-by: David S. Miller <davem@davemloft.net>
As noticed by Mikulas Patocka, the backoff macros don't
completely nop out for UP builds, we still get a
branch always and a delay slot nop.
Fix this by making the branch to the backoff spin loop
selective, then we can nop out the spin loop completely.
Signed-off-by: David S. Miller <davem@davemloft.net>
The majority of this patch was created by the following script:
***
ASM=arch/sparc/include/asm
mkdir -p $ASM
git mv include/asm-sparc64/ftrace.h $ASM
git rm include/asm-sparc64/*
git mv include/asm-sparc/* $ASM
sed -ie 's/asm-sparc64/asm/g' $ASM/*
sed -ie 's/asm-sparc/asm/g' $ASM/*
***
The rest was an update of the top-level Makefile to use sparc
for header files when sparc64 is being build.
And a small fixlet to pick up the correct unistd.h from
sparc64 code.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
David S. Miller