Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC 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>
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: <20191023095607.GE3016@techsingularity.net>
 <20191023102737.32274-1-mhocko@kernel.org>
 <20191023102737.32274-3-mhocko@kernel.org>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <a3510617-fd23-9f90-3c40-700bcb0f353c@redhat.com>
Date:   Wed, 23 Oct 2019 10:56:30 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.2
MIME-Version: 1.0
In-Reply-To: <20191023102737.32274-3-mhocko@kernel.org>
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10/23/19 6:27 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> 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.
>
> 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 therefore putting
> a bound on the number of pages on the free_list sounds like a reasonable
> tradeoff.
>
> The new output will simply tell
> [...]
> Node    6, zone   Normal, type      Movable >100000 >100000 >100000 >1000=
00  41019  31560  23996  10054   3229    983    648
>
> instead of
> Node    6, zone   Normal, type      Movable 399568 294127 221558 102119  =
41019  31560  23996  10054   3229    983    648
>
> The limit has been chosen arbitrary and it is a subject of a future
> change should there be a need for that.
>
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/vmstat.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4e885ecd44d1..762034fc3b83 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1386,8 +1386,25 @@ static void pagetypeinfo_showfree_print(struct seq=
_file *m,
> =20
>  =09=09=09area =3D &(zone->free_area[order]);
> =20
> -=09=09=09list_for_each(curr, &area->free_list[mtype])
> +=09=09=09list_for_each(curr, &area->free_list[mtype]) {
>  =09=09=09=09freecount++;
> +=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
> +=09=09=09=09 * sufficient
> +=09=09=09=09 */
> +=09=09=09=09if (freecount > 100000) {
> +=09=09=09=09=09seq_printf(m, ">%6lu ", freecount);
> +=09=09=09=09=09spin_unlock_irq(&zone->lock);
> +=09=09=09=09=09cond_resched();
> +=09=09=09=09=09spin_lock_irq(&zone->lock);
> +=09=09=09=09=09continue;
list_for_each() is a for loop. The continue statement will just iterate
the rests with the possibility that curr will be stale. Should we use
goto to jump after the seq_print() below?
> +=09=09=09=09}
> +=09=09=09}
>  =09=09=09seq_printf(m, "%6lu ", freecount);
>  =09=09}
>  =09=09seq_putc(m, '\n');

Cheers,
Longman