linux-kernel - Re: [PATCH v4 1/2] mm: refactor __vunmap() to avoid duplicated call to find_vm

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Date:   Wed, 17 Apr 2019 14:58:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guroan@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH v4 1/2] mm: refactor __vunmap() to avoid duplicated call
 to find_vm_area()

On Wed, 17 Apr 2019 12:40:01 -0700 Roman Gushchin <guroan@...il.com> wrote:

> __vunmap() calls find_vm_area() twice without an obvious reason:
> first directly to get the area pointer, second indirectly by calling
> remove_vm_area(), which is again searching for the area.
> 
> To remove this redundancy, let's split remove_vm_area() into
> __remove_vm_area(struct vmap_area *), which performs the actual area
> removal, and remove_vm_area(const void *addr) wrapper, which can
> be used everywhere, where it has been used before.
> 
> On my test setup, I've got 5-10% speed up on vfree()'ing 1000000
> of 4-pages vmalloc blocks.
> 
> Perf report before:
>   22.64%  cat      [kernel.vmlinux]  [k] free_pcppages_bulk
>   10.30%  cat      [kernel.vmlinux]  [k] __vunmap
>    9.80%  cat      [kernel.vmlinux]  [k] find_vmap_area
>    8.11%  cat      [kernel.vmlinux]  [k] vunmap_page_range
>    4.20%  cat      [kernel.vmlinux]  [k] __slab_free
>    3.56%  cat      [kernel.vmlinux]  [k] __list_del_entry_valid
>    3.46%  cat      [kernel.vmlinux]  [k] smp_call_function_many
>    3.33%  cat      [kernel.vmlinux]  [k] kfree
>    3.32%  cat      [kernel.vmlinux]  [k] free_unref_page
> 
> Perf report after:
>   23.01%  cat      [kernel.kallsyms]  [k] free_pcppages_bulk
>    9.46%  cat      [kernel.kallsyms]  [k] __vunmap
>    9.15%  cat      [kernel.kallsyms]  [k] vunmap_page_range
>    6.17%  cat      [kernel.kallsyms]  [k] __slab_free
>    5.61%  cat      [kernel.kallsyms]  [k] kfree
>    4.86%  cat      [kernel.kallsyms]  [k] bad_range
>    4.67%  cat      [kernel.kallsyms]  [k] free_unref_page_commit
>    4.24%  cat      [kernel.kallsyms]  [k] __list_del_entry_valid
>    3.68%  cat      [kernel.kallsyms]  [k] free_unref_page
>    3.65%  cat      [kernel.kallsyms]  [k] __list_add_valid
>    3.19%  cat      [kernel.kallsyms]  [k] __purge_vmap_area_lazy
>    3.10%  cat      [kernel.kallsyms]  [k] find_vmap_area
>    3.05%  cat      [kernel.kallsyms]  [k] rcu_cblist_dequeue
> 
> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2068,6 +2068,24 @@ struct vm_struct *find_vm_area(const void *addr)
>  	return NULL;
>  }
>  
> +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> +{
> +	struct vm_struct *vm = va->vm;
> +
> +	might_sleep();

Where might __remove_vm_area() sleep?

>From a quick scan I'm only seeing vfree(), and that has the
might_sleep_if(!in_interrupt()).

So perhaps we can remove this...

> +	spin_lock(&vmap_area_lock);
> +	va->vm = NULL;
> +	va->flags &= ~VM_VM_AREA;
> +	va->flags |= VM_LAZY_FREE;
> +	spin_unlock(&vmap_area_lock);
> +
> +	kasan_free_shadow(vm);
> +	free_unmap_vmap_area(va);
> +
> +	return vm;
> +}
> +