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@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx> > >> >> > --- > >> >> > 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(). 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(). While it appears unlikely, there are bad conditions which can result in cond_resched() being avoided. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
