On 06/10/2014 10:19 PM, Joonsoo Kim wrote:
Richard Yao reported a month ago that his system have a trouble
with vmap_area_lock contention during performance analysis
by /proc/meminfo. Andrew asked why his analysis checks /proc/meminfo
stressfully, but he didn't answer it.
https://lkml.org/lkml/2014/4/10/416
Although I'm not sure that this is right usage or not, there is a solution
reducing vmap_area_lock contention with no side-effect. That is just
to use rcu list iterator in get_vmalloc_info().
rcu can be used in this function because all RCU protocol is already
respected by writers, since Nick Piggin commit db64fe02258f1507e13fe5
("mm: rewrite vmap layer") back in linux-2.6.28
While rcu list traversal over the vmap_area_list is safe, this may
arrive at different results than the spinlocked version. The rcu list
traversal version will not be a 'snapshot' of a single, valid instant
of the entire vmap_area_list, but rather a potential amalgam of
different list states.
This is because the vmap_area_list can continue to change during
list traversal.
Regards,
Peter Hurley
Specifically :
insertions use list_add_rcu(),
deletions use list_del_rcu() and kfree_rcu().
Note the rb tree is not used from rcu reader (it would not be safe),
only the vmap_area_list has full RCU protection.
Note that __purge_vmap_area_lazy() already uses this rcu protection.
rcu_read_lock();
list_for_each_entry_rcu(va, &vmap_area_list, list) {
if (va->flags & VM_LAZY_FREE) {
if (va->va_start < *start)
*start = va->va_start;
if (va->va_end > *end)
*end = va->va_end;
nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
list_add_tail(&va->purge_list, &valist);
va->flags |= VM_LAZY_FREEING;
va->flags &= ~VM_LAZY_FREE;
}
}
rcu_read_unlock();
v2: add more commit description from Eric
[eduma...@google.com: add more commit description]
Reported-by: Richard Yao <r...@gentoo.org>
Acked-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo....@lge.com>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f64632b..fdbb116 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2690,14 +2690,14 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
prev_end = VMALLOC_START;
- spin_lock(&vmap_area_lock);
+ rcu_read_lock();
if (list_empty(&vmap_area_list)) {
vmi->largest_chunk = VMALLOC_TOTAL;
goto out;
}
- list_for_each_entry(va, &vmap_area_list, list) {
+ list_for_each_entry_rcu(va, &vmap_area_list, list) {
unsigned long addr = va->va_start;
/*
@@ -2724,7 +2724,7 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
vmi->largest_chunk = VMALLOC_END - prev_end;
out:
- spin_unlock(&vmap_area_lock);
+ rcu_read_unlock();
}
#endif
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