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> >> >> > --- >> >> > Â mm/vmscan.c | Â Â4 ++++ >> >> > Â 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, >> >> > Â Â unsigned long balanced = 0; >> >> > Â Â bool all_zones_ok = true; >> >> > >> >> > + Â /* If kswapd has been running too long, just sleep */ >> >> > + Â if (need_resched()) >> >> > + Â Â Â Â Â 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: Âneed_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. ÂGenerally 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. ÂChris Mason first suspected the hang was >> > a kswapd rescheduling problem a while ago. ÂWe tried putting >> > cond_rescheds() in several places in the vmscan code, but to no avail. >> >> Is it a result of Âtest 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. ÂThe 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? > > -- > Mel Gorman > SUSE Labs > -- Kind regards, Minchan Kim -- 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