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 12:17:37 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        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>
Subject: Re: [RFC PATCH 2/2] mm, vmstat: reduce zone->lock holding time by
 /proc/pagetypeinfo

On 10/23/19 12:10 PM, Michal Hocko wrote:
> On Wed 23-10-19 10:56:30, 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);
>>> +					spin_unlock_irq(&zone->lock);
>>> +					cond_resched();
>>> +					spin_lock_irq(&zone->lock);
>>> +					continue;
>> 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?
> You are right. Kinda brown paper back material. Sorry about that. What
> about this on top?
> --- 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 762034fc3b83..c156ce24a322 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1383,11 +1383,11 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  			unsigned long freecount = 0;
>  			struct free_area *area;
>  			struct list_head *curr;
> +			bool overflow = false;
>  
>  			area = &(zone->free_area[order]);
>  
>  			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
> @@ -1397,15 +1397,15 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
>  				 * of pages in this order should be more than
>  				 * sufficient
>  				 */
> -				if (freecount > 100000) {
> -					seq_printf(m, ">%6lu ", freecount);
> +				if (++freecount >= 100000) {
> +					overflow = true;
>  					spin_unlock_irq(&zone->lock);
>  					cond_resched();
>  					spin_lock_irq(&zone->lock);
> -					continue;
> +					break;
>  				}
>  			}
> -			seq_printf(m, "%6lu ", freecount);
> +			seq_printf(m, "%s%6lu ", overflow ? ">" : "", freecount);
>  		}
>  		seq_putc(m, '\n');
>  	}
>
Yes, that looks good to me. There is still a small chance that the
description will be a bit off if it is exactly 100,000. However, it is
not a big deal and I can live with that.

Thanks,
Longman