Re: [PATCH] mm/mm_init.c: remove spinlock in early_pfn_to_nid()

["Yajun Deng" <yajun.deng@linux.dev>]

June 14, 2023 7:53 PM, "Mike Rapoport" <rppt@kernel.org> wrote:

> Hi,
> 
> On Wed, Jun 14, 2023 at 11:28:32AM +0000, Yajun Deng wrote:
> 
>> June 14, 2023 7:09 PM, "Greg KH" <gregkh@linuxfoundation.org> wrote:
>> 
>> On Wed, Jun 14, 2023 at 07:03:24PM +0800, Yajun Deng wrote:
>> 
>> When the system boots, only one cpu is enabled before smp_init().
>> So the spinlock is not needed in most cases, remove it.
>> 
>> Add spinlock in get_nid_for_pfn() because it is after smp_init().
>> 
>> So this is two different things at once in the same patch?
>> 
>> Or are they the same problem and both need to go in to solve it?
>> 
>> And if a spinlock is not needed at early boot, is it really causing any
>> problems?
>> 
>> They are the same problem.
>> I added pr_info in early_pfn_to_nid(), found get_nid_for_pfn() is the only
>> case need to add spinlock.
>> This patch tested on my x86 system.
> 
> Are you sure it'll work on !x86?
>

I'm probably sure of that, although I don't have a !x86 machine.

early_pfn_to_nid() is called in smp_init() and kasan_init() on 
different architectures. If it works well on x86, it'll work on
!x86.

 
>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>> ---
>> drivers/base/node.c | 11 +++++++++--
>> mm/mm_init.c | 18 +++---------------
>> 2 files changed, 12 insertions(+), 17 deletions(-)
>> 
>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>> index 9de524e56307..844102570ff2 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -748,8 +748,15 @@ int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
>> static int __ref get_nid_for_pfn(unsigned long pfn)
>> {
>> #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> - if (system_state < SYSTEM_RUNNING)
>> - return early_pfn_to_nid(pfn);
>> + static DEFINE_SPINLOCK(early_pfn_lock);
>> + int nid;
>> +
>> + if (system_state < SYSTEM_RUNNING) {
>> + spin_lock(&early_pfn_lock);
>> + nid = early_pfn_to_nid(pfn);
>> + spin_unlock(&early_pfn_lock);
>> 
>> Adding an external lock for when you call a function is VERY dangerous
>> as you did not document this anywhere, and there's no way to enforce it
>> properly at all.
>> 
>> I should add a comment before early_pfn_to_nid().
>> 
>> Does your change actually result in any boot time changes? How was this
>> tested?
>> 
>> Just a bit.
> 
> Just a bit tested? Or just a bit of boot time changes?
> For the latter, do you have numbers?
> 

For the latter, the most beneficial function is memmap_init_reserved_pages(),
the boot time changes depending on whether DEFERRED_STRUCT_PAGE_INIT
is defined or not. 

-->memmap_init_reserved_pages()
   -->for_each_reserved_mem_range()
      reserve_bootmem_region()
      -->for()
         init_reserved_page()
         --> early_pfn_to_nid()


If define CONFIG_DEFERRED_STRUCT_PAGE_INIT:

before:
memmap_init_reserved_pages()   1.87 seconds
after:
memmap_init_reserved_pages()   1.27 seconds

32% time reduction.


If not define CONFIG_DEFERRED_STRUCT_PAGE_INIT:

early_pfn_to_nid() is called by few, 
boot time didn't change.


By the way, this machine has 190GB RAM.

> --
> Sincerely yours,
> Mike.