linux-kernel - Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by /proc/pagetypeinfo

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Date:   Wed, 23 Oct 2019 11:21:00 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>
Cc:     Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <guro@...com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jann Horn <jannh@...gle.com>, Song Liu <songliubraving@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo

On 10/23/19 10:56 AM, Waiman Long wrote:
> On 10/23/19 6:27 AM, Michal Hocko wrote:
>> From: Michal Hocko <mhocko@...e.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 >100000  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@...ux-foundation.org>
>> Signed-off-by: Michal Hocko <mhocko@...e.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,
>>  
>>  			area = &(zone->free_area[order]);
>>  
>> -			list_for_each(curr, &area->free_list[mtype])
>> +			list_for_each(curr, &area->free_list[mtype]) {
>>  				freecount++;
>> +				/*
>> +				 * Cap the free_list iteration because it might
>> +				 * be really large and we are under a spinlock
>> +				 * so a long time spent here could trigger a
>> +				 * hard lockup detector. Anyway this is a
>> +				 * debugging tool so knowing there is a handful
>> +				 * of pages in this order should be more than
>> +				 * sufficient
>> +				 */
>> +				if (freecount > 100000) {
>> +					seq_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