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Subject: Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo
From:   Waiman Long <longman@redhat.com>
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>
 <a3510617-fd23-9f90-3c40-700bcb0f353c@redhat.com>
Organization: Red Hat
Message-ID: <c5d19ad0-db65-e592-490a-3ba73f81296b@redhat.com>
Date:   Wed, 23 Oct 2019 11:21:00 -0400
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On 10/23/19 10:56 AM, Waiman Long wrote:
> 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 >100=
000  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 se=
q_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);

It will print ">100001" which seems a bit awk and will be incorrect if
it is exactly 100001. Could you just hardcode ">100000" into seq_printf()?

Cheers,
Longman