Re: [PATCH] mm: swap out anonymous page regardless of laptop_mode

[Luigi Semenzato <semenzato@google.com>]

No problem at all---as I mentioned, we stopped using laptop_mode, so
this is no longer an issue for us.

I should be able to test the patch for you in the next 2-3 days.  I
will let you know if I run into problems.

Thanks!
Luigi

On Mon, Jan 7, 2013 at 11:53 PM, Minchan Kim <minchan@kernel.org> wrote:
> Hi Luigi,
>
> Sorry for really really late response.
> Today I have a time to look at this problem and it seems to found the problem.
> By your help, I can reprocude this problem easily on my KVM machine and this
> patch solves the problem.
>
> Could you test below patch? Although this patch is based on recent mmotm,
> I guess you can apply it easily to 3.4.
>
> From f74fdf644bec3e7875d245154db953b47b6c9594 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Tue, 8 Jan 2013 16:23:31 +0900
> Subject: [PATCH] mm: swap out anonymous page regardless of laptop_mode
>
> Recently, Luigi reported there are lots of free swap space when
> OOM happens. It's easily reproduced on zram-over-swap, where
> many instance of memory hogs are running and laptop_mode is enabled.
>
> Luigi reported there was no problem when he disabled laptop_mode.
> The problem when I investigate problem is following as.
>
> try_to_free_pages disable may_writepage if laptop_mode is enabled.
> shrink_page_list adds lots of anon pages in swap cache by
> add_to_swap, which makes pages Dirty and rotate them to head of
> inactive LRU without pageout. If it is repeated, inactive anon LRU
> is full of Dirty and SwapCache pages.
>
> In case of that, isolate_lru_pages fails because it try to isolate
> clean page due to may_writepage == 0.
>
> may_writepage could be 1 only if total_scanned is higher than
> writeback_threshold in do_try_to_free_pages but unfortunately,
> VM can't isolate anon pages from inactive anon lru list by
> above reason and we already reclaimed all file-backed pages.
> So it ends up OOM killing.
>
> This patch makes may_writepage could be set when shrink_inactive_list
> encounters SwapCachePage from tail of inactive anon LRU.
> What it means that anon LRU list is short and memory pressure
> is severe so it would be better to swap out that pages by sacrificing
> the power rather than OOM killing.
>
> Reported-by: Luigi Semenzato <semenzato@google.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/vmscan.c |   13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ff869d2..7397a6b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1102,7 +1102,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>                 prefetchw_prev_lru_page(page, src, flags);
>
>                 VM_BUG_ON(!PageLRU(page));
> -
> +retry:
>                 switch (__isolate_lru_page(page, mode)) {
>                 case 0:
>                         nr_pages = hpage_nr_pages(page);
> @@ -1112,6 +1112,17 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>                         break;
>
>                 case -EBUSY:
> +                       /*
> +                        * If VM encounters PageSwapCache from inactive LRU,
> +                        * it means we havd to swap out those pages regardless
> +                        * of laptop_mode for preventing OOM kill.
> +                        */
> +                       if ((mode & ISOLATE_CLEAN) && PageSwapCache(page) &&
> +                               !PageActive(page)) {
> +                               mode &= ~ISOLATE_CLEAN;
> +                               sc->may_writepage = 1;
> +                               goto retry;
> +                       }
>                         /* else it is being freed elsewhere */
>                         list_move(&page->lru, src);
>                         continue;
> --
> 1.7.9.5
>
>
> On Thu, Nov 29, 2012 at 11:31:46AM -0800, Luigi Semenzato wrote:
>> Oh well, I found the problem, it's laptop_mode.  We keep it on by
>> default.  When I turn it off, I can allocate as fast as I can, and no
>> OOMs happen until swap is exhausted.
>>
>> I don't think this is a desirable behavior even for laptop_mode, so if
>> anybody wants to help me debug it (or wants my help in debugging it)
>> do let me know.
>>
>> Thanks!
>> Luigi
>>
>> On Thu, Nov 29, 2012 at 10:46 AM, Luigi Semenzato <semenzato@google.com> wrote:
>> > Minchan:
>> >
>> > I tried your suggestion to move the call to wake_all_kswapd from after
>> > "restart:" to after "rebalance:".  The behavior is still similar, but
>> > slightly improved.  Here's what I see.
>> >
>> > Allocating as fast as I can: 1.5 GB of the 3 GB of zram swap are used,
>> > then OOM kills happen, and the system ends up with 1 GB swap used, 2
>> > unused.
>> >
>> > Allocating 10 MB/s: some kills happen when only 1 to 1.5 GB are used,
>> > and continue happening while swap fills up.  Eventually swap fills up
>> > completely.  This is better than before (could not go past about 1 GB
>> > of swap used), but there are too many kills too early.  I would like
>> > to see no OOM kills until swap is full or almost full.
>> >
>> > Allocating 20 MB/s: almost as good as with 10 MB/s, but more kills
>> > happen earlier, and not all swap space is used (400 MB free at the
>> > end).
>> >
>> > This is with 200 processes using 20 MB each, and 2:1 compression ratio.
>> >
>> > So it looks like kswapd is still not aggressive enough in pushing
>> > pages out.  What's the best way of changing that?  Play around with
>> > the watermarks?
>> >
>> > Incidentally, I also tried removing the min_filelist_kbytes hacky
>> > patch, but, as usual, the system thrashes so badly that it's
>> > impossible to complete any experiment.  I set it to a lower minimum
>> > amount of free file pages, 10 MB instead of the 50 MB which we use
>> > normally, and I could run with some thrashing, but I got the same
>> > results.
>> >
>> > Thanks!
>> > Luigi
>> >
>> >
>> > On Wed, Nov 28, 2012 at 4:31 PM, Luigi Semenzato <semenzato@google.com> wrote:
>> >> I am beginning to understand why zram appears to work fine on our x86
>> >> systems but not on our ARM systems.  The bottom line is that swapping
>> >> doesn't work as I would expect when allocation is "too fast".
>> >>
>> >> In one of my tests, opening 50 tabs simultaneously in a Chrome browser
>> >> on devices with 2 GB of RAM and a zram-disk of 3 GB (uncompressed), I
>> >> was observing that on the x86 device all of the zram swap space was
>> >> used before OOM kills happened, but on the ARM device I would see OOM
>> >> kills when only about 1 GB (out of 3) was swapped out.
>> >>
>> >> I wrote a simple program to understand this behavior.  The program
>> >> (called "hog") allocates memory and fills it with a mix of
>> >> incompressible data (from /dev/urandom) and highly compressible data
>> >> (1's, just to avoid zero pages) in a given ratio.  The memory is never
>> >> touched again.
>> >>
>> >> It turns out that if I don't limit the allocation speed, I see
>> >> premature OOM kills also on the x86 device.  If I limit the allocation
>> >> to 10 MB/s, the premature OOM kills stop happening on the x86 device,
>> >> but still happen on the ARM device.  If I further limit the allocation
>> >> speed to 5 Mb/s, the premature OOM kills disappear also from the ARM
>> >> device.
>> >>
>> >> I have noticed a few time constants in the MM whose value is not well
>> >> explained, and I am wondering if the code is tuned for some ideal
>> >> system that doesn't behave like ours (considering, for instance, that
>> >> zram is much faster than swapping to a disk device, but it also uses
>> >> more CPU).  If this is plausible, I am wondering if anybody has
>> >> suggestions for changes that I could try out to obtain a better
>> >> behavior with a higher allocation speed.
>> >>
>> >> Thanks!
>> >> Luigi
>>
>> --
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>
> --
> Kind regards,
> Minchan Kim

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