From patchwork Sun Oct 28 06:13:33 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [1/2] sparc64: Fix cpu strand yielding. Date: Sat, 27 Oct 2012 20:13:33 -0000 From: David Miller X-Patchwork-Id: 194648 Message-Id: <20121028.021333.1563327700655419230.davem@davemloft.net> To: sparclinux@vger.kernel.org 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 --- arch/sparc/include/asm/backoff.h | 5 ++++- arch/sparc/include/asm/processor_64.h | 5 ++++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/arch/sparc/include/asm/backoff.h b/arch/sparc/include/asm/backoff.h index db3af0d..64b077b 100644 --- a/arch/sparc/include/asm/backoff.h +++ b/arch/sparc/include/asm/backoff.h @@ -13,7 +13,10 @@ #define BACKOFF_SPIN(reg, tmp, label) \ mov reg, tmp; \ -88: brnz,pt tmp, 88b; \ +88: rd %ccr, %g0; \ + rd %ccr, %g0; \ + rd %ccr, %g0; \ + brnz,pt tmp, 88b; \ sub tmp, 1, tmp; \ set BACKOFF_LIMIT, tmp; \ cmp reg, tmp; \ diff --git a/arch/sparc/include/asm/processor_64.h b/arch/sparc/include/asm/processor_64.h index 4e5a483..9865634 100644 --- a/arch/sparc/include/asm/processor_64.h +++ b/arch/sparc/include/asm/processor_64.h @@ -196,7 +196,10 @@ extern unsigned long get_wchan(struct task_struct *task); #define KSTK_EIP(tsk) (task_pt_regs(tsk)->tpc) #define KSTK_ESP(tsk) (task_pt_regs(tsk)->u_regs[UREG_FP]) -#define cpu_relax() barrier() +#define cpu_relax() asm volatile("rd %%ccr, %%g0\n\t" \ + "rd %%ccr, %%g0\n\t" \ + "rd %%ccr, %%g0" \ + ::: "memory") /* Prefetch support. This is tuned for UltraSPARC-III and later. * UltraSPARC-I will treat these as nops, and UltraSPARC-II has