Re: [PATCH 3/3] Provide control over unmapped pages

Balbir Singh
2010-12-01T05:23:01Z

* Balbir Singh <balbir@linux.vnet.ibm.com> [2010-12-01 10:46:32]:

> * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2010-12-01 09:14:13]:
>
> > > Provide control using zone_reclaim() and a boot parameter. The
> > > code reuses functionality from zone_reclaim() to isolate unmapped
> > > pages and reclaim them as a priority, ahead of other mapped pages.
> > >
> > > Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> > > ---
> > > include/linux/swap.h | 5 ++-
> > > mm/page_alloc.c | 7 +++--
> > > mm/vmscan.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++-
> > > 3 files changed, 79 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > > index eba53e7..78b0830 100644
> > > --- a/include/linux/swap.h
> > > +++ b/include/linux/swap.h
> > > @@ -252,11 +252,12 @@ extern int vm_swappiness;
> > > extern int remove_mapping(struct address_space *mapping, struct page *page);
> > > extern long vm_total_pages;
> > >
> > > -#ifdef CONFIG_NUMA
> > > -extern int zone_reclaim_mode;
> > > extern int sysctl_min_unmapped_ratio;
> > > extern int sysctl_min_slab_ratio;
> > > extern int zone_reclaim(struct zone *, gfp_t, unsigned int);
> > > +extern bool should_balance_unmapped_pages(struct zone *zone);
> > > +#ifdef CONFIG_NUMA
> > > +extern int zone_reclaim_mode;
> > > #else
> > > #define zone_reclaim_mode 0
> > > static inline int zone_reclaim(struct zone *z, gfp_t mask, unsigned int order)
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 62b7280..4228da3 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1662,6 +1662,9 @@ zonelist_scan:
> > > unsigned long mark;
> > > int ret;
> > >
> > > + if (should_balance_unmapped_pages(zone))
> > > + wakeup_kswapd(zone, order);
> > > +
> >
> > You don't have to add extra branch into fast path.
> >
> >
> > > mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> > > if (zone_watermark_ok(zone, order, mark,
> > > classzone_idx, alloc_flags))
> > > @@ -4136,10 +4139,10 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
> > >
> > > zone->spanned_pages = size;
> > > zone->present_pages = realsize;
> > > -#ifdef CONFIG_NUMA
> > > - zone->node = nid;
> > > zone->min_unmapped_pages = (realsize*sysctl_min_unmapped_ratio)
> > > / 100;
> > > +#ifdef CONFIG_NUMA
> > > + zone->node = nid;
> > > zone->min_slab_pages = (realsize * sysctl_min_slab_ratio) / 100;
> > > #endif
> > > zone->name = zone_names[j];
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 0ac444f..98950f4 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -145,6 +145,21 @@ static DECLARE_RWSEM(shrinker_rwsem);
> > > #define scanning_global_lru(sc) (1)
> > > #endif
> > >
> > > +static unsigned long balance_unmapped_pages(int priority, struct zone *zone,
> > > + struct scan_control *sc);
> > > +static int unmapped_page_control __read_mostly;
> > > +
> > > +static int __init unmapped_page_control_parm(char *str)
> > > +{
> > > + unmapped_page_control = 1;
> > > + /*
> > > + * XXX: Should we tweak swappiness here?
> > > + */
> > > + return 1;
> > > +}
> > > +__setup("unmapped_page_control", unmapped_page_control_parm);
> > > +
> > > +
> > > static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
> > > struct scan_control *sc)
> > > {
> > > @@ -2223,6 +2238,12 @@ loop_again:
> > > shrink_active_list(SWAP_CLUSTER_MAX, zone,
> > > &sc, priority, 0);
> > >
> > > + /*
> > > + * We do unmapped page balancing once here and once
> > > + * below, so that we don't lose out
> > > + */
> > > + balance_unmapped_pages(priority, zone, &sc);
> >
> > You can't invoke any reclaim from here. It is in zone balancing detection
> > phase. It mean your code reclaim pages from zones which has lots free pages too.
>
> The goal is to check not only for zone_watermark_ok, but also to see
> if unmapped pages are way higher than expected values.
>
> >
> > > +
> > > if (!zone_watermark_ok_safe(zone, order,
> > > high_wmark_pages(zone), 0, 0)) {
> > > end_zone = i;
> > > @@ -2258,6 +2279,11 @@ loop_again:
> > > continue;
> > >
> > > sc.nr_scanned = 0;
> > > + /*
> > > + * Balance unmapped pages upfront, this should be
> > > + * really cheap
> > > + */
> > > + balance_unmapped_pages(priority, zone, &sc);
> >
> >
> > This code break page-cache/slab balancing logic. And this is conflict
> > against Nick's per-zone slab effort.
> >
>
> OK, cc'ing Nick for comments.
>
> > Plus, high-order + priority=5 reclaim Simon's case. (see "Free memory never
> > fully used, swapping" threads)
> >
>
> OK, this path should not add to swapping activity, if that is your
> concern.
>
>
> > >
> > > /*
> > > * Call soft limit reclaim before calling shrink_zone.
> > > @@ -2491,7 +2517,8 @@ void wakeup_kswapd(struct zone *zone, int order)
> > > pgdat->kswapd_max_order = order;
> > > if (!waitqueue_active(&pgdat->kswapd_wait))
> > > return;
> > > - if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0))
> > > + if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0) &&
> > > + !should_balance_unmapped_pages(zone))
> > > return;
> > >
> > > trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
> > > @@ -2740,6 +2767,49 @@ zone_reclaim_unmapped_pages(struct zone *zone, struct scan_control *sc,
> > > }
> > >
> > > /*
> > > + * Routine to balance unmapped pages, inspired from the code under
> > > + * CONFIG_NUMA that does unmapped page and slab page control by keeping
> > > + * min_unmapped_pages in the zone. We currently reclaim just unmapped
> > > + * pages, slab control will come in soon, at which point this routine
> > > + * should be called balance cached pages
> > > + */
> > > +static unsigned long balance_unmapped_pages(int priority, struct zone *zone,
> > > + struct scan_control *sc)
> > > +{
> > > + if (unmapped_page_control &&
> > > + (zone_unmapped_file_pages(zone) > zone->min_unmapped_pages)) {
> > > + struct scan_control nsc;
> > > + unsigned long nr_pages;
> > > +
> > > + nsc = *sc;
> > > +
> > > + nsc.swappiness = 0;
> > > + nsc.may_writepage = 0;
> > > + nsc.may_unmap = 0;
> > > + nsc.nr_reclaimed = 0;
> >
> > Don't you need to fill nsc.nr_to_reclaim field?
> >
>
> Yes, since the relcaim code looks at nr_reclaimed, it needs to be 0 at
> every iteration - did I miss something?
>
> > > +
> > > + nr_pages = zone_unmapped_file_pages(zone) -
> > > + zone->min_unmapped_pages;
> > > + /* Magically try to reclaim eighth the unmapped cache pages */
> > > + nr_pages >>= 3;
> >
> > Please don't make magic.
> >
>
> OK, it is a hueristic, how do I use it?
>
> > > +
> > > + zone_reclaim_unmapped_pages(zone, &nsc, nr_pages);
> > > + return nsc.nr_reclaimed;
> > > + }
> > > + return 0;
> > > +}
> > > +
> > > +#define UNMAPPED_PAGE_RATIO 16
> >
> > Please don't make magic ratio.
>
> OK, it is a hueristic, how do I use heuristics - sysctl?
>
> >
> > > +bool should_balance_unmapped_pages(struct zone *zone)
> > > +{
> > > + if (unmapped_page_control &&
> > > + (zone_unmapped_file_pages(zone) >
> > > + UNMAPPED_PAGE_RATIO * zone->min_unmapped_pages))
> > > + return true;
> > > + return false;
> > > +}
> > > +
> > > +/*
> > > * Try to free up some pages from this zone through reclaim.
> > > */
> > > static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> >
> >
> > Hmm....
> >
> > As far as I reviewed, I can't find any reason why this patch works as expected.
> > So, I think cleancache looks promising more than this idea. Have you seen Dan's
> > patch? I would suggested discuss him.
>
> Please try the patch, I've been using it and it works exactly as
> expected for me. kswapd does the balancing and works well. I've posted
> some data as well.
>

My local MTA failed to deliver the message, trying again.

--
Three Cheers,
Balbir