The attached change significantly improves the performance of the LWS-
CAS code in syscall.S.
This allows a number of packages to build (e.g., zeromq3, gtest and
libxs) that previously failed because
slow LWS-CAS performance under contention. In particular, interrupts
taken while the lock was taken
degraded performance significantly.
The change does the following:
1) Disables interrupts around the CAS operation,
2) Removes the lock code on UP systems where it is not needed, and
3) Changes the loads and stores to use the ordered completer, "o", on
PA 2.0. "o" and "ma" with a zero offset are equivalent. The latter
is accepted
on both PA 1.X and 2.0.
The use of ordered loads and stores probably makes no difference on
all existing
hardware, but it seemed pedantically correct. In particular, the CAS
operation must
complete before LDCW lock is released. As written before, a processor
could reorder
the operations.
I don't believe the period interrupts are disabled is long enough to
significantly
increase interrupt latency. For example, the TLB insert code is
longer. Worst
case is a memory fault in the CAS operation.
Signed-off-by: John David Anglin <dave.anglin@xxxxxxxx>
---
diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
index a63bb179..801ba08 100644
--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -589,12 +589,16 @@ cas_nocontend:
# endif
/* ENABLE_LWS_DEBUG */
+ rsm PSW_SM_I, %r0 /* Disable interrupts */
+#ifdef CONFIG_SMP
LDCW 0(%sr2,%r20), %r28 /* Try to acquire the lock */
cmpb,<>,n %r0, %r28, cas_action /* Did we get it? */
cas_wouldblock:
ldo 2(%r0), %r28 /* 2nd case */
+ ssm PSW_SM_I, %r0
b lws_exit /* Contended... */
ldo -EAGAIN(%r0), %r21 /* Spin in userspace */
+#endif
/*
prev = *addr;
@@ -619,15 +623,19 @@ cas_action:
stw %r1, 4(%sr2,%r20)
#endif
/* The load and store could fail */
-1: ldw 0(%sr3,%r26), %r28
+1: ldw,ma 0(%sr3,%r26), %r28
sub,<> %r28, %r25, %r0
-2: stw %r24, 0(%sr3,%r26)
+2: stw,ma %r24, 0(%sr3,%r26)
+#ifdef CONFIG_SMP
/* Free lock */
- stw %r20, 0(%sr2,%r20)
+ stw,ma %r20, 0(%sr2,%r20)
+#endif
#if ENABLE_LWS_DEBUG
/* Clear thread register indicator */
stw %r0, 4(%sr2,%r20)
#endif
+ /* Enable interrupts */
+ ssm PSW_SM_I, %r0
/* Return to userspace, set no error */
b lws_exit
copy %r0, %r21
@@ -639,6 +647,7 @@ cas_action:
#if ENABLE_LWS_DEBUG
stw %r0, 4(%sr2,%r20)
#endif
+ ssm PSW_SM_I, %r0
b lws_exit
ldo -EFAULT(%r0),%r21 /* set errno */
nop
--
John David Anglin dave.anglin@xxxxxxxx
