Re: [PATCH v2] mm/vmalloc: terminate searching since one node found

[Zhaoyang Huang]

On Mon, Jul 17, 2017 at 4:29 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Mon 17-07-17 15:27:31, Zhaoyang Huang wrote:
>> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxxxxxx:wq>
>>
>> It is no need to find the very beginning of the area within
>> alloc_vmap_area, which can be done by judging each node during the process
>>
>> For current approach, the worst case is that the starting node which be found
>> for searching the 'vmap_area_list' is close to the 'vstart', while the final
>> available one is round to the tail(especially for the left branch).
>> This commit have the list searching start at the first available node, which
>> will save the time of walking the rb tree'(1)' and walking the list(2).
>>
>>       vmap_area_root
>>           /      \
>>      tmp_next     U
>>         /   (1)
>>       tmp
>>        /
>>      ...
>>       /
>>     first(current approach)
>>
>> vmap_area_list->...->first->...->tmp->tmp_next
>>                             (2)
>
> This still doesn't answer questions posted for your previous version
> http://lkml.kernel.org/r/20170717070024.GC7397@xxxxxxxxxxxxxx
>
> Please note that is really important to describe _why_ the patch is
> needed. What has changed can be easily read in the diff...
>
I did some test on an ARM64 platform and found that there is no great help nor
regression for vmalloc. By more investigation, I find that the vmalloc area for
64bit arch is too huge to reach the end of the vmap_free_list, which have the
new allocated area just grow up(seems no chance to use the rb tree). I
will try to
find a 32bit platform for more test.

>> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxxxxxx>
>> ---
>>  mm/vmalloc.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 34a1c3e..f833e07 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -459,9 +459,16 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>>
>>               while (n) {
>>                       struct vmap_area *tmp;
>> +                     struct vmap_area *tmp_next;
>>                       tmp = rb_entry(n, struct vmap_area, rb_node);
>> +                     tmp_next = list_next_entry(tmp, list);
>>                       if (tmp->va_end >= addr) {
>>                               first = tmp;
>> +                             if (ALIGN(tmp->va_end, align) + size
>> +                                             < tmp_next->va_start) {
>> +                                     addr = ALIGN(tmp->va_end, align);
>> +                                     goto found;
>> +                             }
>>                               if (tmp->va_start <= addr)
>>                                       break;
>>                               n = n->rb_left;
>> --
>> 1.9.1
>>
>> --
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>
> --
> Michal Hocko
> SUSE Labs