Re: [PATCH v2 3/5] mm: use a dedicated workqueue for the free workers

[Minchan Kim <minchan@kernel.org>]

Hi,

On Wed, Mar 15, 2017 at 05:00:02PM +0800, Aaron Lu wrote:
> Introduce a workqueue for all the free workers so that user can fine
> tune how many workers can be active through sysfs interface: max_active.
> More workers will normally lead to better performance, but too many can
> cause severe lock contention.

Let me ask a question.

How well can workqueue distribute the jobs in multiple CPU?
I don't ask about currency but parallelism.
I guess benefit you are seeing comes from the parallelism and
for your goal, unbound wq should spawn a thread per cpu and
doing the work in every each CPU. does it work?

> 
> Note that since the zone lock is global, the workqueue is also global
> for all processes, i.e. if we set 8 to max_active, we will have at most
> 8 workers active for all processes that are doing munmap()/exit()/etc.
> 
> Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> ---
>  mm/memory.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 001c7720d773..19b25bb5f45b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -253,6 +253,19 @@ static void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
>  	__tlb_reset_range(tlb);
>  }
>  
> +static struct workqueue_struct *batch_free_wq;
> +static int __init batch_free_wq_init(void)
> +{
> +	batch_free_wq = alloc_workqueue("batch_free_wq",
> +					WQ_UNBOUND | WQ_SYSFS, 0);
> +	if (!batch_free_wq) {
> +		pr_warn("failed to create workqueue batch_free_wq\n");
> +		return -ENOMEM;
> +	}
> +	return 0;
> +}
> +subsys_initcall(batch_free_wq_init);
> +
>  static void tlb_flush_mmu_free_batches(struct mmu_gather_batch *batch_start,
>  				       bool free_batch_page)
>  {
> @@ -306,7 +319,7 @@ static void tlb_flush_mmu_free(struct mmu_gather *tlb)
>  		batch_free->batch_start = tlb->local.next;
>  		INIT_WORK(&batch_free->work, batch_free_work);
>  		list_add_tail(&batch_free->list, &tlb->worker_list);
> -		queue_work(system_unbound_wq, &batch_free->work);
> +		queue_work(batch_free_wq, &batch_free->work);
>  
>  		tlb->batch_count = 0;
>  		tlb->local.next = NULL;
> -- 
> 2.7.4
> 
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