Re: [PATCH v1 2/2] mm: add priority threshold to __purge_vmap_area_lazy()

[Joel Fernandes]

On Thu, Jan 24, 2019 at 12:56:48PM +0100, Uladzislau Rezki (Sony) wrote:
> commit 763b218ddfaf ("mm: add preempt points into
> __purge_vmap_area_lazy()")
> 
> introduced some preempt points, one of those is making an
> allocation more prioritized over lazy free of vmap areas.
> 
> Prioritizing an allocation over freeing does not work well
> all the time, i.e. it should be rather a compromise.
> 
> 1) Number of lazy pages directly influence on busy list length
> thus on operations like: allocation, lookup, unmap, remove, etc.
> 
> 2) Under heavy stress of vmalloc subsystem i run into a situation
> when memory usage gets increased hitting out_of_memory -> panic
> state due to completely blocking of logic that frees vmap areas
> in the __purge_vmap_area_lazy() function.
> 
> Establish a threshold passing which the freeing is prioritized
> back over allocation creating a balance between each other.

I'm a bit concerned that this will introduce the latency back if vmap_lazy_nr
is greater than half of lazy_max_pages(). Which IIUC will be more likely if
the number of CPUs is large.

In fact, when vmap_lazy_nr is high, that's when the latency will be the worst
so one could say that that's when you *should* reschedule since the frees are
taking too long and hurting real-time tasks.

Could this be better solved by tweaking lazy_max_pages() such that purging is
more aggressive?

Another approach could be to detect the scenario you brought up (allocations
happening faster than free), somehow, and avoid a reschedule?

thanks,

 - Joel

> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
>  mm/vmalloc.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index fb4fb5fcee74..abe83f885069 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -661,23 +661,27 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
>  	struct llist_node *valist;
>  	struct vmap_area *va;
>  	struct vmap_area *n_va;
> -	bool do_free = false;
> +	int resched_threshold;
>  
>  	lockdep_assert_held(&vmap_purge_lock);
>  
>  	valist = llist_del_all(&vmap_purge_list);
> +	if (unlikely(valist == NULL))
> +		return false;
> +
> +	/*
> +	 * TODO: to calculate a flush range without looping.
> +	 * The list can be up to lazy_max_pages() elements.
> +	 */
>  	llist_for_each_entry(va, valist, purge_list) {
>  		if (va->va_start < start)
>  			start = va->va_start;
>  		if (va->va_end > end)
>  			end = va->va_end;
> -		do_free = true;
>  	}
>  
> -	if (!do_free)
> -		return false;
> -
>  	flush_tlb_kernel_range(start, end);
> +	resched_threshold = (int) lazy_max_pages() << 1;
>  
>  	spin_lock(&vmap_area_lock);
>  	llist_for_each_entry_safe(va, n_va, valist, purge_list) {
> @@ -685,7 +689,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
>  
>  		__free_vmap_area(va);
>  		atomic_sub(nr, &vmap_lazy_nr);
> -		cond_resched_lock(&vmap_area_lock);
> +
> +		if (atomic_read(&vmap_lazy_nr) < resched_threshold)
> +			cond_resched_lock(&vmap_area_lock);
>  	}
>  	spin_unlock(&vmap_area_lock);
>  	return true;
> -- 
> 2.11.0
>