Re: [PATCH] mm: Skip the reserved bootmem for compaction

[Rong Qianfeng <11065417@vivo.com>]

Hi Mel,

在 2024/9/4 19:13, Mel Gorman 写道:
> On Mon, Sep 02, 2024 at 08:24:43PM +0800, Rong Qianfeng wrote:
>> Reserved pages are basically non-lru pages. This kind of memory can't be
>> used as migration sources and targets, skip it can bring some performance
>> benefits.
>>
>> Because some drivers may also use PG_reserved, we just set PB_migrate_skip
>> for those clustered reserved bootmem during memory initialization.
>>
>> Signed-off-by: Rong Qianfeng <rongqianfeng@vivo.com>
> I'm not convinced the savings due to skipping a few pages during the scan
> would justify the additional code. There would have to be a large number
> of reserved pages scattered throughout the zone to make a difference and
> even that situation would be a big surprise. I'm not even sure this can be
> explicitly tested unless you artifically create reserved pages throughout the
> zone, which would not be convincing, or know if a driver that exhibits such
> behaviour in which case my first question is -- what is that driver doing?!?

Thanks for taking the time to reply.

At first I thought that there was not much PageReserved pages, but when I
looked at the memory initialization code, I found that no-map pages were
also marked as PageReserved.  On mobile platforms, there is a lot of no-map
pages (for example, ARM64 MT6991 no-map pages has 1065MB).  These
pages are usually used by various hardware subsystems such as modem.  So
I think it makes sense to skip these pages.


//no-map and  reserved memory marked as PageReserved
static void __init memmap_init_reserved_pages(void)
{
...
     for_each_mem_region(region) {
...
         if (memblock_is_nomap(region))
             reserve_bootmem_region(start, end, nid);  //for no-map memory

         memblock_set_node(start, end, &memblock.reserved, nid);
     }

     for_each_reserved_mem_region(region) {
         if (!memblock_is_reserved_noinit(region)) {
...
             reserve_bootmem_region(start, end, nid); //for reserved memory
         }
     }

}

Best Regards,
Qianfeng