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Date: Thu, 21 Mar 2013 15:26:02 +0000
From: Mel Gorman <mgorman@...e.de>
To: Michal Hocko <mhocko@...e.cz>
Cc: Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Rik van Riel <riel@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Johannes Weiner <hannes@...xchg.org>,
dormando <dormando@...ia.net>,
Satoru Moriya <satoru.moriya@....com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/10] mm: vmscan: Flatten kswapd priority loop
On Thu, Mar 21, 2013 at 03:54:58PM +0100, Michal Hocko wrote:
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> > mm/vmscan.c | 86 ++++++++++++++++++++++++++++++-------------------------------
> > 1 file changed, 42 insertions(+), 44 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 182ff15..279d0c2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2625,8 +2625,11 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
> > /*
> > * kswapd shrinks the zone by the number of pages required to reach
> > * the high watermark.
> > + *
> > + * Returns true if kswapd scanned at least the requested number of
> > + * pages to reclaim.
>
> Maybe move the comment about not rising priority in such case here to be
> clear what the return value means. Without that the return value could
> be misinterpreted that kswapd_shrink_zone succeeded in shrinking might
> be not true.
I moved the comment.
> Or maybe even better, leave the void there and add bool *raise_priority
> argument here so the decision and raise_priority are at the same place.
>
The priority is raised if kswapd failed to reclaim from any of the unbalanced
zone. If raise_priority is moved inside kswapd_shrink_zone then it can
only take one zone into account.
> > */
> > -static void kswapd_shrink_zone(struct zone *zone,
> > +static bool kswapd_shrink_zone(struct zone *zone,
> > struct scan_control *sc,
> > unsigned long lru_pages)
> > {
> > @@ -2646,6 +2649,8 @@ static void kswapd_shrink_zone(struct zone *zone,
> >
> > if (nr_slab == 0 && !zone_reclaimable(zone))
> > zone->all_unreclaimable = 1;
> > +
> > + return sc->nr_scanned >= sc->nr_to_reclaim;
> > }
> >
> > /*
> [...]
> > @@ -2803,8 +2805,16 @@ loop_again:
> >
> > if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
> > !zone_balanced(zone, testorder,
> > - balance_gap, end_zone))
> > - kswapd_shrink_zone(zone, &sc, lru_pages);
> > + balance_gap, end_zone)) {
> > + /*
> > + * There should be no need to raise the
> > + * scanning priority if enough pages are
> > + * already being scanned that that high
>
> s/that that/that/
>
Fixed
> > + * watermark would be met at 100% efficiency.
> > + */
> > + if (kswapd_shrink_zone(zone, &sc, lru_pages))
> > + raise_priority = false;
> > + }
> >
> > /*
> > * If we're getting trouble reclaiming, start doing
> > @@ -2839,46 +2849,33 @@ loop_again:
> > pfmemalloc_watermark_ok(pgdat))
> > wake_up(&pgdat->pfmemalloc_wait);
> >
> > - if (pgdat_balanced(pgdat, order, *classzone_idx)) {
> > - pgdat_is_balanced = true;
> > - break; /* kswapd: all done */
> > - }
> > -
> > /*
> > - * We do this so kswapd doesn't build up large priorities for
> > - * example when it is freeing in parallel with allocators. It
> > - * matches the direct reclaim path behaviour in terms of impact
> > - * on zone->*_priority.
> > + * Fragmentation may mean that the system cannot be rebalanced
> > + * for high-order allocations in all zones. If twice the
> > + * allocation size has been reclaimed and the zones are still
> > + * not balanced then recheck the watermarks at order-0 to
> > + * prevent kswapd reclaiming excessively. Assume that a
> > + * process requested a high-order can direct reclaim/compact.
> > */
> > - if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
> > - break;
> > - } while (--sc.priority >= 0);
> > + if (order && sc.nr_reclaimed >= 2UL << order)
> > + order = sc.order = 0;
> >
> > -out:
> > - if (!pgdat_is_balanced) {
> > - cond_resched();
> > + /* Check if kswapd should be suspending */
> > + if (try_to_freeze() || kthread_should_stop())
> > + break;
> >
> > - try_to_freeze();
> > + /* If no reclaim progress then increase scanning priority */
> > + if (sc.nr_reclaimed - nr_reclaimed == 0)
> > + raise_priority = true;
> >
> > /*
> > - * Fragmentation may mean that the system cannot be
> > - * rebalanced for high-order allocations in all zones.
> > - * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
> > - * it means the zones have been fully scanned and are still
> > - * not balanced. For high-order allocations, there is
> > - * little point trying all over again as kswapd may
> > - * infinite loop.
> > - *
> > - * Instead, recheck all watermarks at order-0 as they
> > - * are the most important. If watermarks are ok, kswapd will go
> > - * back to sleep. High-order users can still perform direct
> > - * reclaim if they wish.
> > + * Raise priority if scanning rate is too low or there was no
> > + * progress in reclaiming pages
> > */
> > - if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > - order = sc.order = 0;
> > -
> > - goto loop_again;
> > - }
> > + if (raise_priority || sc.nr_reclaimed - nr_reclaimed == 0)
>
> (sc.nr_reclaimed - nr_reclaimed == 0) is redundant because you already
> set raise_priority above in that case.
>
I removed the redundant check.
> > + sc.priority--;
> > + } while (sc.priority >= 0 &&
> > + !pgdat_balanced(pgdat, order, *classzone_idx));
> >
> > /*
> > * If kswapd was reclaiming at a higher order, it has the option of
> > @@ -2907,6 +2904,7 @@ out:
> > compact_pgdat(pgdat, order);
> > }
> >
> > +out:
> > /*
> > * Return the order we were reclaiming at so prepare_kswapd_sleep()
> > * makes a decision on the order we were last reclaiming at. However,
>
> It looks OK otherwise but I have to think some more as balance_pgdat is
> still tricky, albeit less then it was before so this is definitely
> progress.
>
Thanks.
--
Mel Gorman
SUSE Labs
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