Re: Oops in __free_pages_ok (pre7-1) (Long) (backtrace)

[Linus Torvalds <torvalds@transmeta.com>]

On Thu, 4 May 2000, Rajagopal Ananthanarayanan wrote:
> 
> I did some testing of this patch with dbench.
> The kernel starts shooting processes down pretty quickly
> ("VM: killing process XXX") on a 2 CPU 64MB system,
> with nothing but dbench (8 clients). A concurrently
> running vmstat shows very low free memory with some swapping,
> and the buffer space remaining around 50MB.

Ok. The page locking patch should not change any swap behaviour at all, so
this behaviour is likely to be due to the pageout changes by Rik.

(Oh, the page locking might cause another part of the vmscanning logic to
temporarily ignore a page because it is locked, but that should be a very
small second-order effect compared to the "big picture" changes in how
much to page out).

Rik, I think the kswapd logic is wrong, and I suspect you made it
worsewhen you added the while-loop. The problem looks like that while
kswapd is working on one zone, it will entirely ignore any other zones. I
think the logic should be more like

	for (;;) {
		int something_to_do = 0;
		pgdat = pgdat_list;
		while (pgdat) {
			for(i = 0; i < MAX_NR_ZONES; i++) {
				zone = pgdat->node_zones+ i;
				if (!zone->size || !zone->zone_wake_kswapd)
					continue;
				something_to_do = 1;
				do_try_to_free_pages(GFP_KSWAPD, zone);
			}
			run_task_queue(&tq_disk);
			pgdat = pgdat->node_next;
		}
		if (something_to_do) {
			if (tsk->need_resched)
				schedule();
			continue;
		}
		tsk->state = TASK_INTERRUPTIBLE;
		interruptible_sleep_on(&kswapd_wait);
	}

See? This has two changes to the current logic:
 - it is more "balanced" on the do_try_to_free_pages(), ie it calls it for
   different zones instead of repeating one zone until no longer needed.
 - it continues to do this until no zone needs balancing any more, unlike
   the old one that could easily lose kswapd wakeup-requests and just do
   one zone.

What do you think? I suspect that the added do-loop in pre7 just made the
"lost wakeups" problem worse by concentrating on one zone for a longer
while and thus more likely to lose wakeups for lower zones (because it
already looked at those).

There might be other details like this lurking, but this looks like a good
first try. Ananth, willing to give it a whirl?

			Linus

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