lkml.org 
[lkml]   [2011]   [May]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep
    From
    Date
    On Tue, 2011-05-17 at 11:38 +0100, Mel Gorman wrote:
    > On Tue, May 17, 2011 at 08:50:44AM +0900, Minchan Kim wrote:
    > > 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;
    > > }
    > >
    >
    > This makes some sense for the exit path but if one or more of the
    > shrinkers takes a very long time without sleeping (extremely long
    > list searches for example) then kswapd will not call cond_resched()
    > between shrinkers and still consume a lot of CPU.
    >
    > > >
    > > > 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?
    > >
    >
    > Which will leave kswapd runnable instead of going to sleep but
    > guarantees a scheduling point. Lets see if the problem is that
    > cond_resched is being missed although if this was the case then patch
    > 4 would truly be a no-op but Colin has already reported that patch 1 on
    > its own didn't fix his problem. If the problem is sandybridge-specific
    > where kswapd remains runnable and consuming large amounts of CPU in
    > turbo mode then we know that there are other cond_resched() decisions
    > that will need to be revisited.
    >
    > Colin or James, would you be willing to test with patch 1 from this
    > series and Minchan's patch below? Thanks.

    Yes, but unfortunately I'm on the road at the moment. I won't get back
    to the laptop showing the problem until late on Tuesday (24th). If it
    works for Colin, I'd assume it's OK.

    James


    > > 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
    >




    \
     
     \ /
      Last update: 2011-05-18 06:11    [W:0.028 / U:1.936 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site