Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, May 16, 2011 at 1:21 PM, James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
> On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote:
>> (2011/05/13 23:03), Mel Gorman wrote:
>> > Under constant allocation pressure, kswapd can be in the situation where
>> > sleeping_prematurely() will always return true even if kswapd has been
>> > running a long time. Check if kswapd needs to be scheduled.
>> >
>> > Signed-off-by: Mel Gorman<mgorman@suse.de>
>> > ---
>> >   mm/vmscan.c |    4 ++++
>> >   1 files changed, 4 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index af24d1e..4d24828 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>> >     unsigned long balanced = 0;
>> >     bool all_zones_ok = true;
>> >
>> > +   /* If kswapd has been running too long, just sleep */
>> > +   if (need_resched())
>> > +           return false;
>> > +
>>
>> Hmm... I don't like this patch so much. because this code does
>>
>> - don't sleep if kswapd got context switch at shrink_inactive_list
>
> This isn't entirely true:  need_resched() will be false, so we'll follow
> the normal path for determining whether to sleep or not, in effect
> leaving the current behaviour unchanged.
>
>> - sleep if kswapd didn't
>
> This also isn't entirely true: whether need_resched() is true at this
> point depends on a whole lot more that whether we did a context switch
> in shrink_inactive. It mostly depends on how long we've been running
> without giving up the CPU.  Generally that will mean we've been round
> the shrinker loop hundreds to thousands of times without sleeping.
>
>> It seems to be semi random behavior.
>
> Well, we have to do something.  Chris Mason first suspected the hang was
> a kswapd rescheduling problem a while ago.  We tried putting
> cond_rescheds() in several places in the vmscan code, but to no avail.

Is it a result of  test with patch of Hannes(ie, !pgdat_balanced)?

If it isn't, it would be nop regardless of putting cond_reshed at vmscan.c.
Because, although we complete zone balancing, kswapd doesn't sleep as
pgdat_balance returns wrong result. And at last VM calls
balance_pgdat. In this case, balance_pgdat returns without any work as
kswap couldn't find zones which have not enough free pages and goto
out. kswapd could repeat this work infinitely. So you don't have a
chance to call cond_resched.

But if your test was with Hanne's patch, I am very curious how come
kswapd consumes CPU a lot.

> The need_resched() in sleeping_prematurely() seems to be about the best
> option.  The other option might be just to put a cond_resched() in
> kswapd_try_to_sleep(), but that will really have about the same effect.

I don't oppose it but before that, I think we have to know why kswapd
consumes CPU a lot although we applied Hannes' patch.

>
> James
>
>
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-- 
Kind regards,
Minchan Kim

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