Messages in this thread | | | From | Muchun Song <> | Date | Thu, 10 Dec 2020 20:14:18 +0800 | Subject | Re: [External] Re: [PATCH v8 12/12] mm/hugetlb: Optimize the code with the help of the compiler |
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On Thu, Dec 10, 2020 at 7:39 PM Oscar Salvador <osalvador@suse.de> wrote: > > On 2020-12-10 04:55, Muchun Song wrote: > > We cannot optimize if a "struct page" crosses page boundaries. If > > it is true, we can optimize the code with the help of a compiler. > > When free_vmemmap_pages_per_hpage() returns zero, most functions are > > optimized by the compiler. > > As I said earlier, I would squash this patch with patch#10 and > remove the !is_power_of_2 check in hugetlb_vmemmap_init and leave > only the check for the boot parameter. > That should be enough.
Yeah, you are right. I just want the compiler to do optimization.
> > > static inline bool is_hugetlb_free_vmemmap_enabled(void) > > { > > - return hugetlb_free_vmemmap_enabled; > > + return hugetlb_free_vmemmap_enabled && > > + is_power_of_2(sizeof(struct page)); > > Why? hugetlb_free_vmemmap_enabled can only become true > if the is_power_of_2 check succeeds in early_hugetlb_free_vmemmap_param. > The "is_power_of_2" check here can go. > > > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h > > index 0a1c0d33a316..5f5e90c81cd2 100644 > > --- a/mm/hugetlb_vmemmap.h > > +++ b/mm/hugetlb_vmemmap.h > > @@ -21,7 +21,7 @@ void free_huge_page_vmemmap(struct hstate *h, struct > > page *head); > > */ > > static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate > > *h) > > { > > - return h->nr_free_vmemmap_pages; > > + return h->nr_free_vmemmap_pages && is_power_of_2(sizeof(struct > > page)); > > If hugetlb_free_vmemmap_enabled is false, hugetlb_vmemmap_init() leaves > h->nr_free_vmemmap_pages unset to 0, so no need for the is_power_of_2 > check here.
Yeah, you are right. But if we do this check can make the code simple.
For example, here is a code snippet.
void func(void) { if (free_vmemmap_pages_per_hpage()) return; /* Do something */ }
With this patch, the func will be optimized to null when is_power_of_2 returns false.
void func(void) { }
Without this patch, the compiler cannot do this optimization.
Thanks.
> > > -- > Oscar Salvador > SUSE L3
-- Yours, Muchun
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