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

[Waiman Long <longman@redhat.com>]

On 10/23/19 12:17 PM, Waiman Long wrote:
> 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@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 >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@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,
>>>>  
>>>>  			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.

Alternatively, you can do

if (++freecount > 100000) {
        :
    freecount--;
    break;
}

Cheers,
Longman