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 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. 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. > > James > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxxx ÂFor 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> > -- Kind regards, Minchan Kim -- 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
