Re: [PATCH] Revert "mm: remove __GFP_NO_KSWAPD"

Mel Gorman
2012-11-21T15:08:57Z

On Tue, Nov 20, 2012 at 10:38:45AM -0500, Josh Boyer wrote:
> On Fri, Nov 16, 2012 at 3:06 PM, Mel Gorman <mgorman@suse.de> wrote:
> > On Fri, Nov 16, 2012 at 02:14:47PM -0500, Josh Boyer wrote:
> >> On Mon, Nov 12, 2012 at 6:37 AM, Mel Gorman <mgorman@suse.de> wrote:
> >> > With "mm: vmscan: scale number of pages reclaimed by reclaim/compaction
> >> > based on failures" reverted, Zdenek Kabelac reported the following
> >> >
> >> > Hmm, so it's just took longer to hit the problem and observe
> >> > kswapd0 spinning on my CPU again - it's not as endless like before -
> >> > but still it easily eats minutes - it helps to turn off Firefox
> >> > or TB (memory hungry apps) so kswapd0 stops soon - and restart
> >> > those apps again. (And I still have like >1GB of cached memory)
> >> >
> >> > kswapd0 R running task 0 30 2 0x00000000
> >> > ffff8801331efae8 0000000000000082 0000000000000018 0000000000000246
> >> > ffff880135b9a340 ffff8801331effd8 ffff8801331effd8 ffff8801331effd8
> >> > ffff880055dfa340 ffff880135b9a340 00000000331efad8 ffff8801331ee000
> >> > Call Trace:
> >> > [<ffffffff81555bf2>] preempt_schedule+0x42/0x60
> >> > [<ffffffff81557a95>] _raw_spin_unlock+0x55/0x60
> >> > [<ffffffff81192971>] put_super+0x31/0x40
> >> > [<ffffffff81192a42>] drop_super+0x22/0x30
> >> > [<ffffffff81193b89>] prune_super+0x149/0x1b0
> >> > [<ffffffff81141e2a>] shrink_slab+0xba/0x510
> >> >
> >> > The sysrq+m indicates the system has no swap so it'll never reclaim
> >> > anonymous pages as part of reclaim/compaction. That is one part of the
> >> > problem but not the root cause as file-backed pages could also be reclaimed.
> >> >
> >> > The likely underlying problem is that kswapd is woken up or kept awake
> >> > for each THP allocation request in the page allocator slow path.
> >> >
> >> > If compaction fails for the requesting process then compaction will be
> >> > deferred for a time and direct reclaim is avoided. However, if there
> >> > are a storm of THP requests that are simply rejected, it will still
> >> > be the the case that kswapd is awake for a prolonged period of time
> >> > as pgdat->kswapd_max_order is updated each time. This is noticed by
> >> > the main kswapd() loop and it will not call kswapd_try_to_sleep().
> >> > Instead it will loopp, shrinking a small number of pages and calling
> >> > shrink_slab() on each iteration.
> >> >
> >> > The temptation is to supply a patch that checks if kswapd was woken for
> >> > THP and if so ignore pgdat->kswapd_max_order but it'll be a hack and not
> >> > backed up by proper testing. As 3.7 is very close to release and this is
> >> > not a bug we should release with, a safer path is to revert "mm: remove
> >> > __GFP_NO_KSWAPD" for now and revisit it with the view to ironing out the
> >> > balance_pgdat() logic in general.
> >> >
> >> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> >>
> >> Does anyone know if this is queued to go into 3.7 somewhere? I looked
> >> a bit and can't find it in a tree. We have a few reports of Fedora
> >> rawhide users hitting this.
> >>
> >
> > No, because I was waiting to hear if a) it worked and preferably if the
> > alternative "less safe" option worked. This close to release it might be
> > better to just go with the safe option.
>
> We've been tracking it in https://bugzilla.redhat.com/show_bug.cgi?id=866988
> and people say this revert patch doesn't seem to make the issue go away
> fully. Thorsten has created another kernel with the other patch applied
> for testing.
>

There is also a potential accounting bug that could be affecting this.
https://lkml.org/lkml/2012/11/20/613 . NR_FREE_PAGES affects watermark
calculations. If it's drifts too far then processes would keep entering
direct reclaim and waking kswapd even if there is no need to.

--
Mel Gorman
SUSE Labs