* Arjan van de Ven <[EMAIL PROTECTED]> wrote:
> as I said, since the cacheline just got dirtied, the write is just
> half a cycle which is so much in the noise that it really doesn't
> matter.
ok - the patch below is a small modification of Hugh's so that we clear
->break_lock unconditionally. Since this code is not inlined it ought to
have minimal icache impact too.
Ingo
--
lock->break_lock is set when a lock is contended, but cleared only in
cond_resched_lock. Users of need_lockbreak (journal_commit_transaction,
copy_pte_range, unmap_vmas) don't necessarily use cond_resched_lock on it.
So, if the lock has been contended at some time in the past, break_lock
remains set thereafter, and the fastpath keeps dropping lock unnecessarily.
Hanging the system if you make a change like I did, forever restarting a
loop before making any progress. And even users of cond_resched_lock may
well suffer an initial unnecessary lockbreak.
There seems to be no point at which break_lock can be cleared when
unlocking, any point being either too early or too late; but that's okay,
it's only of interest while the lock is held. So clear it whenever the
lock is acquired - and any waiting contenders will quickly set it again.
Additional locking overhead? well, this is only when CONFIG_PREEMPT is on.
Since cond_resched_lock's spin_lock clears break_lock, no need to clear it
itself; and use need_lockbreak there too, preferring optimizer to #ifdefs.
Signed-off-by: Hugh Dickins <[EMAIL PROTECTED]>
Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
--- 2.6.11-bk8/kernel/sched.c 2005-03-11 13:33:09.000000000 +0000
+++ linux/kernel/sched.c 2005-03-11 17:46:50.000000000 +0000
@@ -3753,14 +3753,11 @@ EXPORT_SYMBOL(cond_resched);
*/
int cond_resched_lock(spinlock_t * lock)
{
-#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT)
- if (lock->break_lock) {
- lock->break_lock = 0;
+ if (need_lockbreak(lock)) {
spin_unlock(lock);
cpu_relax();
spin_lock(lock);
}
-#endif
if (need_resched()) {
_raw_spin_unlock(lock);
preempt_enable_no_resched();
--- 2.6.11-bk8/kernel/spinlock.c 2005-03-02 07:38:52.000000000 +0000
+++ linux/kernel/spinlock.c 2005-03-12 22:52:41.000000000 +0000
@@ -187,6 +187,7 @@ void __lockfunc _##op##_lock(locktype##_
cpu_relax(); \
preempt_disable(); \
} \
+ (lock)->break_lock = 0; \
} \
\
EXPORT_SYMBOL(_##op##_lock); \
@@ -209,6 +211,7 @@ unsigned long __lockfunc _##op##_lock_ir
cpu_relax(); \
preempt_disable(); \
} \
+ (lock)->break_lock = 0; \
return flags; \
} \
\
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
