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Date: Fri, 6 May 2022 10:49:53 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Mike Kravetz <mike.kravetz@...cle.com>
Cc: corbet@....net, akpm@...ux-foundation.org, mcgrof@...nel.org,
keescook@...omium.org, yzaikin@...gle.com, osalvador@...e.de,
david@...hat.com, masahiroy@...nel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v9 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap
sysctl
On Thu, May 05, 2022 at 09:48:34AM -0700, Mike Kravetz wrote:
> On 5/5/22 01:02, Muchun Song wrote:
> > On Wed, May 04, 2022 at 08:36:00PM -0700, Mike Kravetz wrote:
> >> On 5/4/22 19:35, Muchun Song wrote:
> >>> On Wed, May 04, 2022 at 03:12:39PM -0700, Mike Kravetz wrote:
> >>>> On 4/29/22 05:18, Muchun Song wrote:
> >>>>> +static void vmemmap_optimize_mode_switch(enum vmemmap_optimize_mode to)
> >>>>> +{
> >>>>> + if (vmemmap_optimize_mode == to)
> >>>>> + return;
> >>>>> +
> >>>>> + if (to == VMEMMAP_OPTIMIZE_OFF)
> >>>>> + static_branch_dec(&hugetlb_optimize_vmemmap_key);
> >>>>> + else
> >>>>> + static_branch_inc(&hugetlb_optimize_vmemmap_key);
> >>>>> + vmemmap_optimize_mode = to;
> >>>>> +}
> >>>>> +
> >>>>> static int __init hugetlb_vmemmap_early_param(char *buf)
> >>>>> {
> >>>>> bool enable;
> >>>>> + enum vmemmap_optimize_mode mode;
> >>>>>
> >>>>> if (kstrtobool(buf, &enable))
> >>>>> return -EINVAL;
> >>>>>
> >>>>> - if (enable)
> >>>>> - static_branch_enable(&hugetlb_optimize_vmemmap_key);
> >>>>> - else
> >>>>> - static_branch_disable(&hugetlb_optimize_vmemmap_key);
> >>>>> + mode = enable ? VMEMMAP_OPTIMIZE_ON : VMEMMAP_OPTIMIZE_OFF;
> >>>>> + vmemmap_optimize_mode_switch(mode);
> >>>>>
> >>>>> return 0;
> >>>>> }
> >>>>> @@ -60,6 +80,8 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
> >>>>> vmemmap_end = vmemmap_addr + (vmemmap_pages << PAGE_SHIFT);
> >>>>> vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> >>>>>
> >>>>> + VM_BUG_ON_PAGE(!vmemmap_pages, head);
> >>>>> +
> >>>>> /*
> >>>>> * The pages which the vmemmap virtual address range [@vmemmap_addr,
> >>>>> * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> >>>>> @@ -69,8 +91,10 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
> >>>>> */
> >>>>> ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> >>>>> GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> >>>>> - if (!ret)
> >>>>> + if (!ret) {
> >>>>> ClearHPageVmemmapOptimized(head);
> >>>>> + static_branch_dec(&hugetlb_optimize_vmemmap_key);
> >>>>> + }
> >>>>>
> >>>>> return ret;
> >>>>> }
> >>>>> @@ -84,6 +108,8 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
> >>>>> if (!vmemmap_pages)
> >>>>> return;
> >>>>>
> >>>>> + static_branch_inc(&hugetlb_optimize_vmemmap_key);
> >>>>
> >>>> Can you explain the reasoning behind doing the static_branch_inc here in free,
> >>>> and static_branch_dec in alloc?
> >>>> IIUC, they may not be absolutely necessary but you could use the count to
> >>>> know how many optimized pages are in use? Or, I may just be missing
> >>>> something.
> >>>>
> >>>
> >>> Partly right. One 'count' is not enough. I have implemented this with similar
> >>> approach in v6 [1]. Except the 'count', we also need a lock to do synchronization.
> >>> However, both count and synchronization are included in static_key_inc/dec
> >>> infrastructure. It is simpler to use static_key_inc/dec directly, right?
> >>>
> >>> [1] https://lore.kernel.org/all/20220330153745.20465-5-songmuchun@bytedance.com/
> >>>
> >>
> >> Sorry, but I am a little confused.
> >>
> >> vmemmap_optimize_mode_switch will static_key_inc to enable and static_key_dec
> >> to disable. In addition each time we optimize (allocate) a hugetlb page after
> >> enabling we will static_key_inc.
> >>
> >> Suppose we have 1 hugetlb page optimized. So static count == 2 IIUC.
> >> The someone turns off optimization via sysctl. static count == 1 ???
> >
> > Definitely right.
> >
> >> If we then add another hugetlb page via nr_hugepages it seems that it
> >> would be optimized as static count == 1. Is that correct? Do we need
> >
> > I'm wrong.
> >
> >> to free all hugetlb pages with optimization before we can add new pages
> >> without optimization?
> >>
> >
> > My bad. I think the following code would fix this.
> >
> > Thanks for your review carefully.
> >
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 5820a681a724..997e192aeed7 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -105,7 +105,7 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
> > unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
> >
> > vmemmap_pages = hugetlb_optimize_vmemmap_pages(h);
> > - if (!vmemmap_pages)
> > + if (!vmemmap_pages || READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
> > return;
> >
> > static_branch_inc(&hugetlb_optimize_vmemmap_key);
> >
>
> If vmemmap_optimize_mode == VMEMMAP_OPTIMIZE_OFF is sufficient for turning
> off optimizations, do we really need to static_branch_inc/dev for each
> hugetlb page?
>
static_branch_inc/dec is necessary since the user could change
vmemmap_optimize_mode to off after the 'if' judgement.
CPU0: CPU1:
// Assume vmemmap_optimize_mode == 1
// and static_key_count == 1
if (vmemmap_optimize_mode == VMEMMAP_OPTIMIZE_OFF)
return;
hugetlb_optimize_vmemmap_handler();
vmemmap_optimize_mode = 0;
static_branch_dec();
// static_key_count == 0
// Enable static_key if necessary
static_branch_inc();
Does this make sense for you?
Thanks.
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