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

Mel Gorman
2012-11-16T20:06:22Z

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.

--
Mel Gorman
SUSE Labs