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Subject: Re: [PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo
To:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, Waiman Long <longman@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>, Roman Gushchin <guro@fb.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Jann Horn <jannh@google.com>, Song Liu <songliubraving@fb.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rafael Aquini <aquini@redhat.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>
References: <20191025072610.18526-1-mhocko@kernel.org>
 <20191025072610.18526-3-mhocko@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <10350bd9-a0ba-9ba9-82a8-99588acf2312@redhat.com>
Date:   Fri, 25 Oct 2019 10:21:28 +0200
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On 25.10.19 09:26, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>=20
> pagetypeinfo_showfree_print is called by zone->lock held in irq mode.
> This is not really nice because it blocks both any interrupts on that
> cpu and the page allocator. On large machines this might even trigger
> the hard lockup detector.
>=20
> Considering the pagetypeinfo is a debugging tool we do not really need
> exact numbers here. The primary reason to look at the outuput is to see
> how pageblocks are spread among different migratetypes and low number of
> pages is much more interesting therefore putting a bound on the number
> of pages on the free_list sounds like a reasonable tradeoff.
>=20
> The new output will simply tell
> [...]
> Node    6, zone   Normal, type      Movable >100000 >100000 >100000 >1000=
00  41019  31560  23996  10054   3229    983    648
>=20
> instead of
> Node    6, zone   Normal, type      Movable 399568 294127 221558 102119  =
41019  31560  23996  10054   3229    983    648
>=20
> The limit has been chosen arbitrary and it is a subject of a future
> change should there be a need for that.
>=20
> While we are at it, also drop the zone lock after each free_list
> iteration which will help with the IRQ and page allocator responsiveness
> even further as the IRQ lock held time is always bound to those 100k
> pages.
>=20
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>   mm/vmstat.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
>=20
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4e885ecd44d1..ddb89f4e0486 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1383,12 +1383,29 @@ static void pagetypeinfo_showfree_print(struct se=
q_file *m,
>   =09=09=09unsigned long freecount =3D 0;
>   =09=09=09struct free_area *area;
>   =09=09=09struct list_head *curr;
> +=09=09=09bool overflow =3D false;
>  =20
>   =09=09=09area =3D &(zone->free_area[order]);
>  =20
> -=09=09=09list_for_each(curr, &area->free_list[mtype])
> -=09=09=09=09freecount++;
> -=09=09=09seq_printf(m, "%6lu ", freecount);
> +=09=09=09list_for_each(curr, &area->free_list[mtype]) {
> +=09=09=09=09/*
> +=09=09=09=09 * Cap the free_list iteration because it might
> +=09=09=09=09 * be really large and we are under a spinlock
> +=09=09=09=09 * so a long time spent here could trigger a
> +=09=09=09=09 * hard lockup detector. Anyway this is a
> +=09=09=09=09 * debugging tool so knowing there is a handful
> +=09=09=09=09 * of pages in this order should be more than

"of this order" ?

> +=09=09=09=09 * sufficient

s/sufficient"/sufficient." ?

> +=09=09=09=09 */
> +=09=09=09=09if (++freecount >=3D 100000) {
> +=09=09=09=09=09overflow =3D true;
> +=09=09=09=09=09break;
> +=09=09=09=09}
> +=09=09=09}
> +=09=09=09seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
> +=09=09=09spin_unlock_irq(&zone->lock);
> +=09=09=09cond_resched();
> +=09=09=09spin_lock_irq(&zone->lock);
>   =09=09}
>   =09=09seq_putc(m, '\n');
>   =09}
>=20

Acked-by: David Hildenbrand <david@redhat.com>

--=20

Thanks,

David / dhildenb