Subject: Re: [PATCH 1/4] mm/vmalloc: allow to call vfree() in atomic context
To: Michal Hocko <mhocko@kernel.org>
References: <20170330102719.13119-1-aryabinin@virtuozzo.com>
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From: Andrey Ryabinin <aryabinin@virtuozzo.com>
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Date: Wed, 5 Apr 2017 13:31:23 +0300
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On 04/04/2017 12:41 PM, Michal Hocko wrote:
> On Thu 30-03-17 17:48:39, Andrey Ryabinin wrote:
>> From: Andrey Ryabinin <aryabinin@virtuozzo.com>
>> Subject: mm/vmalloc: allow to call vfree() in atomic context fix
>>
>> Don't spawn worker if we already purging.
>>
>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> 
> I would rather put this into a separate patch. Ideally with some numners
> as this is an optimization...
> 

It's quite simple optimization and don't think that this deserves to be a separate patch.

But I did some measurements though. With enabled VMAP_STACK=y and NR_CACHED_STACK changed to 0
running fork() 100000 times gives this:

With optimization:

~ # grep try_purge /proc/kallsyms 
ffffffff811d0dd0 t try_purge_vmap_area_lazy
~ # perf stat --repeat 10 -ae workqueue:workqueue_queue_work --filter 'function == 0xffffffff811d0dd0' ./fork

 Performance counter stats for 'system wide' (10 runs):

                15      workqueue:workqueue_queue_work                                     ( +-  0.88% )

       1.615368474 seconds time elapsed                                          ( +-  0.41% )


Without optimization:
~ # grep try_purge /proc/kallsyms 
ffffffff811d0dd0 t try_purge_vmap_area_lazy
~ # perf stat --repeat 10 -ae workqueue:workqueue_queue_work --filter 'function == 0xffffffff811d0dd0' ./fork

 Performance counter stats for 'system wide' (10 runs):

                30      workqueue:workqueue_queue_work                                     ( +-  1.31% )

       1.613231060 seconds time elapsed                                          ( +-  0.38% )


So there is no measurable difference on the test itself, but we queue twice more jobs without this optimization.
It should decrease load of kworkers.


>> ---
>>  mm/vmalloc.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index ea1b4ab..88168b8 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -737,7 +737,8 @@ static void free_vmap_area_noflush(struct vmap_area *va)
>>  	/* After this point, we may free va at any time */
>>  	llist_add(&va->purge_list, &vmap_purge_list);
>>  
>> -	if (unlikely(nr_lazy > lazy_max_pages()))
>> +	if (unlikely(nr_lazy > lazy_max_pages()) &&
>> +	    !mutex_is_locked(&vmap_purge_lock))
>>  		schedule_work(&purge_vmap_work);
>>  }
>>  
>> -- 
>> 2.10.2
>>
>