Re: [PATCH] mm/vmalloc: reduce the number of lazy_max_pages to reduce latency

[Jisheng Zhang]

On Thu, 29 Sep 2016 09:18:18 +0100 Chris Wilson wrote:

> On Thu, Sep 29, 2016 at 03:34:11PM +0800, Jisheng Zhang wrote:
> > On Marvell berlin arm64 platforms, I see the preemptoff tracer report
> > a max 26543 us latency at __purge_vmap_area_lazy, this latency is an
> > awfully bad for STB. And the ftrace log also shows __free_vmap_area
> > contributes most latency now. I noticed that Joel mentioned the same
> > issue[1] on x86 platform and gave two solutions, but it seems no patch
> > is sent out for this purpose.
> > 
> > This patch adopts Joel's first solution, but I use 16MB per core
> > rather than 8MB per core for the number of lazy_max_pages. After this
> > patch, the preemptoff tracer reports a max 6455us latency, reduced to
> > 1/4 of original result.  
> 
> My understanding is that
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 91f44e78c516..3f7c6d6969ac 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -626,7 +626,6 @@ void set_iounmap_nonlazy(void)
>  static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>                                         int sync, int force_flush)
>  {
> -       static DEFINE_SPINLOCK(purge_lock);
>         struct llist_node *valist;
>         struct vmap_area *va;
>         struct vmap_area *n_va;
> @@ -637,12 +636,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>          * should not expect such behaviour. This just simplifies locking for
>          * the case that isn't actually used at the moment anyway.
>          */
> -       if (!sync && !force_flush) {
> -               if (!spin_trylock(&purge_lock))
> -                       return;
> -       } else
> -               spin_lock(&purge_lock);
> -
>         if (sync)
>                 purge_fragmented_blocks_allcpus();
>  
> @@ -667,7 +660,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>                         __free_vmap_area(va);
>                 spin_unlock(&vmap_area_lock);

Hi Chris,

Per my test, the bottleneck now is __free_vmap_area() over the valist, the
iteration is protected with spinlock vmap_area_lock. So the larger lazy max
pages, the longer valist, the bigger the latency.

So besides above patch, we still need to remove vmap_are_lock or replace with
mutex.

Thanks,
Jisheng

>         }
> -       spin_unlock(&purge_lock);
>  }
>  
>  /*
> 
> 
> should now be safe. That should significantly reduce the preempt-disabled
> section, I think.
> -Chris
>