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Message-Id: <20101130142509.4f49d452.akpm@linux-foundation.org>
Date: Tue, 30 Nov 2010 14:25:09 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc: linux-mm@...ck.org, Christoph Lameter <cl@...ux.com>,
linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/3] Provide control over unmapped pages
On Tue, 30 Nov 2010 15:46:31 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 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@...ux.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;
This change will need to be moved into the first patch.
> 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);
gack, this is on the page allocator fastpath, isn't it? So
99.99999999% of the world's machines end up doing a pointless call to a
pointless function which pointlessly tests a pointless global and
pointlessly returns? All because of some whacky KSM thing?
The speed and space overhead of this code should be *zero* if
!CONFIG_UNMAPPED_PAGECACHE_CONTROL and should be minimal if
CONFIG_UNMAPPED_PAGECACHE_CONTROL=y. The way to do the latter is to
inline the test of unmapped_page_control into callers and only if it is
true (and use unlikely(), please) do we call into the KSM gunk.
> --- 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);
aw c'mon guys, everybody knows that when you add a kernel parameter you
document it in Documentation/kernel-parameters.txt.
> 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);
> +
> 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);
More unjustifiable overhead on a commonly-executed codepath.
> /*
> * 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
> + */
The problem I have with this comment is that it uses the term "balance"
without ever defining it. Plus "balance" is already a term which is used
in memory reclaim.
So if you can think up a unique noun then that's good but whether or
not that is done, please describe with great care what that term
actually means in this context.
> +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;
Doing a clone-and-own of a scan_control is novel. What's going on here?
> + nr_pages = zone_unmapped_file_pages(zone) -
> + zone->min_unmapped_pages;
> + /* Magically try to reclaim eighth the unmapped cache pages */
> + nr_pages >>= 3;
> +
> + zone_reclaim_unmapped_pages(zone, &nsc, nr_pages);
> + return nsc.nr_reclaimed;
> + }
> + return 0;
> +}
> +
> +#define UNMAPPED_PAGE_RATIO 16
Well. Giving 16 a name didn't really clarify anything. Attentive
readers will want to know what this does, why 16 was chosen and what
the effects of changing it will be.
> +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;
> +}
> Reviewed-by: Christoph Lameter <cl@...ux.com>
So you're OK with shoving all this flotsam into 100,000,000 cellphones?
This was a pretty outrageous patchset!
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