Re: [PATCH v10 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap sysctl

[Muchun Song <songmuchun@bytedance.com>]

On Tue, May 10, 2022 at 05:39:40PM -0700, Mike Kravetz wrote:
> On 5/10/22 14:30, Mike Kravetz wrote:
> > On 5/8/22 23:27, Muchun Song wrote:
> >> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> >> index 029fb7e26504..917112661b5c 100644
> >> --- a/include/linux/memory_hotplug.h
> >> +++ b/include/linux/memory_hotplug.h
> >> @@ -351,4 +351,13 @@ void arch_remove_linear_mapping(u64 start, u64 size);
> >>  extern bool mhp_supports_memmap_on_memory(unsigned long size);
> >>  #endif /* CONFIG_MEMORY_HOTPLUG */
> >>  
> >> +#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
> >> +bool mhp_memmap_on_memory(void);
> >> +#else
> >> +static inline bool mhp_memmap_on_memory(void)
> >> +{
> >> +	return false;
> >> +}
> >> +#endif
> >> +
> >>  #endif /* __LINUX_MEMORY_HOTPLUG_H */
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index 8605d7eb7f5c..86158eb9da70 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -1617,6 +1617,9 @@ static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
> >>  
> >>  static inline void flush_free_hpage_work(struct hstate *h)
> >>  {
> >> +	if (!hugetlb_optimize_vmemmap_enabled())
> >> +		return;
> >> +
> > 
> > Hi Muchun,
> > 
> > In v9 I was suggesting that we may be able to eliminate the static_branch_inc/dec from the vmemmap free/alloc paths.  With this patch
> > I believe hugetlb_optimize_vmemmap_enabled() is really checking
> > 'has hugetlb vmemmap optimization been enabled' OR 'are there still vmemmap
> > optimized hugetlb pages in the system'.  That may be confusing.
> > 
> 
> Sorry, I forgot about the use of hugetlb_optimize_vmemmap_enabled in
> page_fixed_fake_head.  We need to know if there are any vmemmap optimized
> hugetlb pages in the system in this performance sensitive path.  So,
> static_branch_inc/dec is indeed a good idea.
>

Agree.

> Please disregard my attempt below at removing static_branch_inc/dec.
> 
> I still find the name hugetlb_optimize_vmemmap_enabled a bit confusing as
> it tests two conditions (enabled and pages in use).
>

Right. It tests two conditions.

> You have already 'open coded' just the check for enabled in the routine
> hugetlb_vmemmap_free with:
> 
> 	READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF
> 
> How about having hugetlb_optimize_vmemmap_enabled() just check
> vmemmap_optimize_mode in a manner like above?  Then rename

I'm wondering is it necessary to do this? vmemmap_optimize_mode
is a internal state in hugetlb_vmemmap.c, at leaset now there is
no outside users who care about this.  Open-coding may be not
an issue (I guess)?  If one day someone cares it, maybe it it
the time to do this and rename hugetlb_optimize_vmemmap_enabled()?
I'm not against doing this, just expressing some of my thoughts.
What do you think, Mike?

> hugetlb_optimize_vmemmap_enabled to something like:
> hugetlb_optimized_vmemmap_possible().  Sorry, I can think if a great name.
> 

At least I cannot come up with an appropriate name.
hugetlb_optimize_vmemmap_may_enabled()? It's not easy to come
up with a good name.

Thanks.