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Subject[PATCH 5/12] ksm: keep quiet while list empty
ksm_scan_thread already sleeps in wait_event_interruptible until setting
ksm_run activates it; but if there's nothing on its list to look at, i.e.
nobody has yet said madvise MADV_MERGEABLE, it's a shame to be clocking
up system time and full_scans: ksmd_should_run added to check that too.

And move the mutex_lock out around it: the new counts showed that when
ksm_run is stopped, a little work often got done afterwards, because it
had been read before taking the mutex.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---

mm/ksm.c | 28 ++++++++++++++++++++++------
1 file changed, 22 insertions(+), 6 deletions(-)

--- ksm4/mm/ksm.c 2009-08-02 13:49:59.000000000 +0100
+++ ksm5/mm/ksm.c 2009-08-02 13:50:07.000000000 +0100
@@ -1287,21 +1287,27 @@ static void ksm_do_scan(unsigned int sca
}
}

+static int ksmd_should_run(void)
+{
+ return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
+}
+
static int ksm_scan_thread(void *nothing)
{
set_user_nice(current, 5);

while (!kthread_should_stop()) {
- if (ksm_run & KSM_RUN_MERGE) {
- mutex_lock(&ksm_thread_mutex);
+ mutex_lock(&ksm_thread_mutex);
+ if (ksmd_should_run())
ksm_do_scan(ksm_thread_pages_to_scan);
- mutex_unlock(&ksm_thread_mutex);
+ mutex_unlock(&ksm_thread_mutex);
+
+ if (ksmd_should_run()) {
schedule_timeout_interruptible(
msecs_to_jiffies(ksm_thread_sleep_millisecs));
} else {
wait_event_interruptible(ksm_thread_wait,
- (ksm_run & KSM_RUN_MERGE) ||
- kthread_should_stop());
+ ksmd_should_run() || kthread_should_stop());
}
}
return 0;
@@ -1346,10 +1352,16 @@ int ksm_madvise(struct vm_area_struct *v

int __ksm_enter(struct mm_struct *mm)
{
- struct mm_slot *mm_slot = alloc_mm_slot();
+ struct mm_slot *mm_slot;
+ int needs_wakeup;
+
+ mm_slot = alloc_mm_slot();
if (!mm_slot)
return -ENOMEM;

+ /* Check ksm_run too? Would need tighter locking */
+ needs_wakeup = list_empty(&ksm_mm_head.mm_list);
+
spin_lock(&ksm_mmlist_lock);
insert_to_mm_slots_hash(mm, mm_slot);
/*
@@ -1361,6 +1373,10 @@ int __ksm_enter(struct mm_struct *mm)
spin_unlock(&ksm_mmlist_lock);

set_bit(MMF_VM_MERGEABLE, &mm->flags);
+
+ if (needs_wakeup)
+ wake_up_interruptible(&ksm_thread_wait);
+
return 0;
}


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