On Tue, 2011-05-17 at 11:38 +0100, Mel Gorman wrote:

On Tue, May 17, 2011 at 08:50:44AM +0900, Minchan Kim wrote:

quoted

Don't we have to move cond_resched?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 292582c..633e761 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -231,8 +231,10 @@ unsigned long shrink_slab(struct shrink_control *shrink,
        if (scanned == 0)
                scanned = SWAP_CLUSTER_MAX;

-       if (!down_read_trylock(&shrinker_rwsem))
-               return 1;       /* Assume we'll be able to shrink next time */
+       if (!down_read_trylock(&shrinker_rwsem)) {
+               ret = 1;
+               goto out; /* Assume we'll be able to shrink next time */
+       }

        list_for_each_entry(shrinker, &shrinker_list, list) {
                unsigned long long delta;

@@ -280,12 +282,14 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                        count_vm_events(SLABS_SCANNED, this_scan);
                        total_scan -= this_scan;

-                       cond_resched();
                }

                shrinker->nr += total_scan;
+               cond_resched();
        }
        up_read(&shrinker_rwsem);
+out:
+       cond_resched();
        return ret;
 }

This makes some sense for the exit path but if one or more of the
shrinkers takes a very long time without sleeping (extremely long
list searches for example) then kswapd will not call cond_resched()
between shrinkers and still consume a lot of CPU.

quoted

quoted

balance_pgdat() only calls cond_resched if the zones are not
       balanced. For a high-order allocation that is balanced, it
       checks order-0 again. During that window, order-0 might have
       become unbalanced so it loops again for order-0 and returns
       that was reclaiming for order-0 to kswapd(). It can then find
       that a caller has rewoken kswapd for a high-order and re-enters
       balance_pgdat() without ever have called cond_resched().

If kswapd reclaims order-o followed by high order, it would have a
chance to call cond_resched in shrink_page_list. But if all zones are
all_unreclaimable is set, balance_pgdat could return any work. Okay.
It does make sense.
By your scenario, someone wakes up kswapd with higher order, again.
So re-enters balance_pgdat without ever have called cond_resched.
But if someone wakes up higher order again, we can't have a chance to
call kswapd_try_to_sleep. So your patch effect would be nop, too.

It would be better to put cond_resched after balance_pgdat?

Which will leave kswapd runnable instead of going to sleep but
guarantees a scheduling point. Lets see if the problem is that
cond_resched is being missed although if this was the case then patch
4 would truly be a no-op but Colin has already reported that patch 1 on
its own didn't fix his problem. If the problem is sandybridge-specific
where kswapd remains runnable and consuming large amounts of CPU in
turbo mode then we know that there are other cond_resched() decisions
that will need to be revisited.

Colin or James, would you be willing to test with patch 1 from this
series and Minchan's patch below? Thanks.

Yes, but unfortunately I'm on the road at the moment.  I won't get back
to the laptop showing the problem until late on Tuesday (24th).  If it
works for Colin, I'd assume it's OK.

James

quoted

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 292582c..61c45d0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -2753,6 +2753,7 @@ static int kswapd(void *p)
                if (!ret) {
                        trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
                        order = balance_pgdat(pgdat, order, &classzone_idx);
+                       cond_resched();
                }
        }
        return 0;

quoted

While it appears unlikely, there are bad conditions which can result
in cond_resched() being avoided.

quoted

--
Mel Gorman
SUSE Labs

-- 
Kind regards,
Minchan Kim