On Thu, May 12, 2011 at 07:47:05PM -0500, James Bottomley wrote: > On Fri, 2011-05-13 at 00:15 +0200, Johannes Weiner wrote: > > On Thu, May 12, 2011 at 05:04:41PM -0500, James Bottomley wrote: > > > On Thu, 2011-05-12 at 15:04 -0500, James Bottomley wrote: > > > > Confirmed, I'm afraid ... I can trigger the problem with all three > > > > patches under PREEMPT. It's not a hang this time, it's just kswapd > > > > taking 100% system time on 1 CPU and it won't calm down after I unload > > > > the system. > > > > > > Just on a "if you don't know what's wrong poke about and see" basis, I > > > sliced out all the complex logic in sleeping_prematurely() and, as far > > > as I can tell, it cures the problem behaviour. I've loaded up the > > > system, and taken the tar load generator through three runs without > > > producing a spinning kswapd (this is PREEMPT). I'll try with a > > > non-PREEMPT kernel shortly. > > > > > > What this seems to say is that there's a problem with the complex logic > > > in sleeping_prematurely(). I'm pretty sure hacking up > > > sleeping_prematurely() just to dump all the calculations is the wrong > > > thing to do, but perhaps someone can see what the right thing is ... > > > > I think I see the problem: the boolean logic of sleeping_prematurely() > > is odd. If it returns true, kswapd will keep running. So if > > pgdat_balanced() returns true, kswapd should go to sleep. > > > > This? > > I was going to say this was a winner, but on the third untar run on > non-PREEMPT, I hit the kswapd livelock. It's got much farther than > previous attempts, which all hang on the first run, but I think the > essential problem is still (at least on this machine) that > sleeping_prematurely() is doing too much work for the wakeup storm that > allocators are causing. > > Something that ratelimits the amount of time we spend in the watermark > calculations, like the below (which incorporates your pgdat fix) seems > to be much more stable (I've not run it for three full runs yet, but > kswapd CPU time is way lower so far). > > The heuristic here is that if we're making the calculation more than ten > times in 1/10 of a second, stop and sleep anyway. > Is that heuristic not basically the same as this? 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; + /* If a direct reclaimer woke kswapd within HZ/10, it's premature */ if (remaining) return true; -- 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>