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    SubjectRe: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep
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    On Mon, May 16, 2011 at 7:27 PM, Mel Gorman <mgorman@suse.de> wrote:
    > On Mon, May 16, 2011 at 05:58:59PM +0900, Minchan Kim wrote:
    >> On Mon, May 16, 2011 at 5:45 PM, Mel Gorman <mgorman@suse.de> 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@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.
    >> >>
    >> >
    >> > 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?
    >>
    >
    > Lets see;
    >
    > shrink_page_list() only applies if inactive pages were isolated
    >        which in turn may not happen if all_unreclaimable is set in
    >        shrink_zones(). If for whatver reason, all_unreclaimable is
    >        set on all zones, we can miss calling cond_resched().
    >
    > shrink_slab only applies if we are reclaiming slab pages. If the first
    >        shrinker returns -1, we do not call cond_resched(). If that
    >        first shrinker is dcache and __GFP_FS is not set, direct
    >        reclaimers will not shrink at all. However, if there are
    >        enough of them running or if one of the other shrinkers
    >        is running for a very long time, kswapd could be starved
    >        acquiring the shrinker_rwsem and never reaching the
    >        cond_resched().

    Don't we have to move cond_resched?

    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index 292582c..633e761 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -231,8 +231,10 @@ unsigned long shrink_slab(struct shrink_control *shrink,
    if (scanned == 0)
    scanned = SWAP_CLUSTER_MAX;

    - if (!down_read_trylock(&shrinker_rwsem))
    - return 1; /* Assume we'll be able to shrink next time */
    + if (!down_read_trylock(&shrinker_rwsem)) {
    + ret = 1;
    + goto out; /* Assume we'll be able to shrink next time */
    + }

    list_for_each_entry(shrinker, &shrinker_list, list) {
    unsigned long long delta;
    @@ -280,12 +282,14 @@ unsigned long shrink_slab(struct shrink_control *shrink,
    count_vm_events(SLABS_SCANNED, this_scan);
    total_scan -= this_scan;

    - cond_resched();
    }

    shrinker->nr += total_scan;
    + cond_resched();
    }
    up_read(&shrinker_rwsem);
    +out:
    + cond_resched();
    return ret;
    }

    >
    > balance_pgdat() only calls cond_resched if the zones are not
    >        balanced. For a high-order allocation that is balanced, it
    >        checks order-0 again. During that window, order-0 might have
    >        become unbalanced so it loops again for order-0 and returns
    >        that was reclaiming for order-0 to kswapd(). It can then find
    >        that a caller has rewoken kswapd for a high-order and re-enters
    >        balance_pgdat() without ever have called cond_resched().

    If kswapd reclaims order-o followed by high order, it would have a
    chance to call cond_resched in shrink_page_list. But if all zones are
    all_unreclaimable is set, balance_pgdat could return any work. Okay.
    It does make sense.
    By your scenario, someone wakes up kswapd with higher order, again.
    So re-enters balance_pgdat without ever have called cond_resched.
    But if someone wakes up higher order again, we can't have a chance to
    call kswapd_try_to_sleep. So your patch effect would be nop, too.

    It would be better to put cond_resched after balance_pgdat?

    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index 292582c..61c45d0 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -2753,6 +2753,7 @@ static int kswapd(void *p)
    if (!ret) {
    trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
    order = balance_pgdat(pgdat, order, &classzone_idx);
    + cond_resched();
    }
    }
    return 0;
    >
    > While it appears unlikely, there are bad conditions which can result
    > in cond_resched() being avoided.

    >
    > --
    > Mel Gorman
    > SUSE Labs
    >



    --
    Kind regards,
    Minchan Kim
    --
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