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. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>