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

Josh Boyer
2012-11-20T15:38:47Z

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.

At least I think that is the latest status from the bug. Hopefully the
commenters will chime in.

josh