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

[Minchan Kim]

On Mon, May 16, 2011 at 5:45 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> On Mon, May 16, 2011 at 02:04:00PM +0900, Minchan Kim wrote:
>> On Mon, May 16, 2011 at 1:21 PM, James Bottomley
>> <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxx>
>> >> > ---
>> >> > Â mm/vmscan.c | Â Â4 ++++
>> >> > Â 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,
>> >> > Â Â unsigned long balanced = 0;
>> >> > Â Â bool all_zones_ok = true;
>> >> >
>> >> > + Â /* If kswapd has been running too long, just sleep */
>> >> > + Â if (need_resched())
>> >> > + Â Â Â Â Â 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: Âneed_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. ÂGenerally 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. ÂChris Mason first suspected the hang was
>> > a kswapd rescheduling problem a while ago. ÂWe tried putting
>> > cond_rescheds() in several places in the vmscan code, but to no avail.
>>
>> Is it a result of Âtest 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. ÂThe 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.
>>
>
> Because it's still possible for processes to allocate pages at the same
> rate kswapd is freeing them leading to a situation where kswapd does not
> consider the zone balanced for prolonged periods of time.

We have cond_resched in shrink_page_list, shrink_slab and balance_pgdat.
So I think kswapd can be scheduled out although it's scheduled in
after a short time as task scheduled also need page reclaim. Although
all task in system need reclaim, kswapd cpu 99% consumption is a
natural result, I think.
Do I miss something?

>
> --
> Mel Gorman
> SUSE Labs
>



-- 
Kind regards,
Minchan Kim
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