Messages in this thread | | | Date | Tue, 17 May 2011 09:48:59 +0900 | Subject | Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep | From | Minchan Kim <> |
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On Tue, May 17, 2011 at 8:50 AM, Minchan Kim <minchan.kim@gmail.com> wrote: > 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.
Typo : without any work.
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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