Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, May 16, 2011 at 7:27 PM, Mel Gorman <mgorman@suse.de> wrote:
> On Mon, May 16, 2011 at 05:58:59PM +0900, Minchan Kim wrote:
>> On Mon, May 16, 2011 at 5:45 PM, Mel Gorman <mgorman@suse.de> wrote:
>> > On Mon, May 16, 2011 at 02:04:00PM +0900, Minchan Kim wrote:
>> >> On Mon, May 16, 2011 at 1:21 PM, James Bottomley
>> >> <James.Bottomley@hansenpartnership.com> wrote:
>> >> > On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote:
>> >> >> (2011/05/13 23:03), Mel Gorman wrote:
>> >> >> > Under constant allocation pressure, kswapd can be in the situation where
>> >> >> > sleeping_prematurely() will always return true even if kswapd has been
>> >> >> > running a long time. Check if kswapd needs to be scheduled.
>> >> >> >
>> >> >> > Signed-off-by: Mel Gorman<mgorman@suse.de>
>> >> >> > ---
>> >> >> >   mm/vmscan.c |    4 ++++
>> >> >> >   1 files changed, 4 insertions(+), 0 deletions(-)
>> >> >> >
>> >> >> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> >> >> > index af24d1e..4d24828 100644
>> >> >> > --- a/mm/vmscan.c
>> >> >> > +++ b/mm/vmscan.c
>> >> >> > @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>> >> >> >     unsigned long balanced = 0;
>> >> >> >     bool all_zones_ok = true;
>> >> >> >
>> >> >> > +   /* If kswapd has been running too long, just sleep */
>> >> >> > +   if (need_resched())
>> >> >> > +           return false;
>> >> >> > +
>> >> >>
>> >> >> Hmm... I don't like this patch so much. because this code does
>> >> >>
>> >> >> - don't sleep if kswapd got context switch at shrink_inactive_list
>> >> >
>> >> > This isn't entirely true:  need_resched() will be false, so we'll follow
>> >> > the normal path for determining whether to sleep or not, in effect
>> >> > leaving the current behaviour unchanged.
>> >> >
>> >> >> - sleep if kswapd didn't
>> >> >
>> >> > This also isn't entirely true: whether need_resched() is true at this
>> >> > point depends on a whole lot more that whether we did a context switch
>> >> > in shrink_inactive. It mostly depends on how long we've been running
>> >> > without giving up the CPU.  Generally that will mean we've been round
>> >> > the shrinker loop hundreds to thousands of times without sleeping.
>> >> >
>> >> >> It seems to be semi random behavior.
>> >> >
>> >> > Well, we have to do something.  Chris Mason first suspected the hang was
>> >> > a kswapd rescheduling problem a while ago.  We tried putting
>> >> > cond_rescheds() in several places in the vmscan code, but to no avail.
>> >>
>> >> Is it a result of  test with patch of Hannes(ie, !pgdat_balanced)?
>> >>
>> >> If it isn't, it would be nop regardless of putting cond_reshed at vmscan.c.
>> >> Because, although we complete zone balancing, kswapd doesn't sleep as
>> >> pgdat_balance returns wrong result. And at last VM calls
>> >> balance_pgdat. In this case, balance_pgdat returns without any work as
>> >> kswap couldn't find zones which have not enough free pages and goto
>> >> out. kswapd could repeat this work infinitely. So you don't have a
>> >> chance to call cond_resched.
>> >>
>> >> But if your test was with Hanne's patch, I am very curious how come
>> >> kswapd consumes CPU a lot.
>> >>
>> >> > The need_resched() in sleeping_prematurely() seems to be about the best
>> >> > option.  The other option might be just to put a cond_resched() in
>> >> > kswapd_try_to_sleep(), but that will really have about the same effect.
>> >>
>> >> I don't oppose it but before that, I think we have to know why kswapd
>> >> consumes CPU a lot although we applied Hannes' patch.
>> >>
>> >
>> > Because it's still possible for processes to allocate pages at the same
>> > rate kswapd is freeing them leading to a situation where kswapd does not
>> > consider the zone balanced for prolonged periods of time.
>>
>> We have cond_resched in shrink_page_list, shrink_slab and balance_pgdat.
>> So I think kswapd can be scheduled out although it's scheduled in
>> after a short time as task scheduled also need page reclaim. Although
>> all task in system need reclaim, kswapd cpu 99% consumption is a
>> natural result, I think.
>> Do I miss something?
>>
>
> Lets see;
>
> shrink_page_list() only applies if inactive pages were isolated
>        which in turn may not happen if all_unreclaimable is set in
>        shrink_zones(). If for whatver reason, all_unreclaimable is
>        set on all zones, we can miss calling cond_resched().
>
> shrink_slab only applies if we are reclaiming slab pages. If the first
>        shrinker returns -1, we do not call cond_resched(). If that
>        first shrinker is dcache and __GFP_FS is not set, direct
>        reclaimers will not shrink at all. However, if there are
>        enough of them running or if one of the other shrinkers
>        is running for a very long time, kswapd could be starved
>        acquiring the shrinker_rwsem and never reaching the
>        cond_resched().

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;
 }


>
> 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?

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;

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

>
> --
> Mel Gorman
> SUSE Labs
>



-- 
Kind regards,
Minchan Kim