linux-kernel - Re: [PATCH] vmalloc: use rcu list iterator to reduce vmap_area

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Date:	Thu, 29 May 2014 09:17:22 -0400
From:	Richard Yao <ryao@...too.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] vmalloc: use rcu list iterator to reduce vmap_area_lock contention

I do not have a way of tracing it. I meant to reply when I did, but that has not changed. That being said, I like this patch.

On May 29, 2014, at 2:22 AM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> Richard Yao reported a month ago that his system have a trouble
> with vmap_area_lock contention during performance analysis
> by /proc/meminfo. Andrew asked why his analysis checks /proc/meminfo
> stressfully, but he didn't answer it.
> 
> https://lkml.org/lkml/2014/4/10/416
> 
> Although I'm not sure that this is right usage or not, there is a solution
> reducing vmap_area_lock contention with no side-effect. That is just
> to use rcu list iterator in get_vmalloc_info(). This function only needs
> values on vmap_area structure, so we don't need to grab a spinlock.
> 
> Reported-by: Richard Yao <ryao@...too.org>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index f64632b..fdbb116 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2690,14 +2690,14 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
> 
>    prev_end = VMALLOC_START;
> 
> -    spin_lock(&vmap_area_lock);
> +    rcu_read_lock();
> 
>    if (list_empty(&vmap_area_list)) {
>        vmi->largest_chunk = VMALLOC_TOTAL;
>        goto out;
>    }
> 
> -    list_for_each_entry(va, &vmap_area_list, list) {
> +    list_for_each_entry_rcu(va, &vmap_area_list, list) {
>        unsigned long addr = va->va_start;
> 
>        /*
> @@ -2724,7 +2724,7 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
>        vmi->largest_chunk = VMALLOC_END - prev_end;
> 
> out:
> -    spin_unlock(&vmap_area_lock);
> +    rcu_read_unlock();
> }
> #endif
> 
> -- 
> 1.7.9.5
> 
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