linux-kernel - Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

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Date:   Thu, 11 Oct 2018 07:59:32 +0530
From:   Arun KS <arunks@...eaurora.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        iamjoonsoo.kim@....com, gregkh@...uxfoundation.org,
        osalvador@...e.de, malat@...ian.org,
        kirill.shutemov@...ux.intel.com, jrdr.linux@...il.com,
        yasu.isimatu@...il.com, mgorman@...hsingularity.net,
        aaron.lu@...el.com, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, vatsa@...eaurora.org,
        vinmenon@...eaurora.org, getarunks@...il.com
Subject: Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

On 2018-10-10 23:03, Michal Hocko wrote:
> On Wed 10-10-18 22:26:41, Arun KS wrote:
>> On 2018-10-10 21:00, Vlastimil Babka wrote:
>> > On 10/5/18 10:10 AM, Arun KS wrote:
>> > > When free pages are done with higher order, time spend on
>> > > coalescing pages by buddy allocator can be reduced. With
>> > > section size of 256MB, hot add latency of a single section
>> > > shows improvement from 50-60 ms to less than 1 ms, hence
>> > > improving the hot add latency by 60%. Modify external
>> > > providers of online callback to align with the change.
>> > >
>> > > Signed-off-by: Arun KS <arunks@...eaurora.org>
>> >
>> > [...]
>> >
>> > > @@ -655,26 +655,44 @@ void __online_page_free(struct page *page)
>> > >  }
>> > >  EXPORT_SYMBOL_GPL(__online_page_free);
>> > >
>> > > -static void generic_online_page(struct page *page)
>> > > +static int generic_online_page(struct page *page, unsigned int order)
>> > >  {
>> > > -	__online_page_set_limits(page);
>> >
>> > This is now not called anymore, although the xen/hv variants still do
>> > it. The function seems empty these days, maybe remove it as a followup
>> > cleanup?
>> >
>> > > -	__online_page_increment_counters(page);
>> > > -	__online_page_free(page);
>> > > +	__free_pages_core(page, order);
>> > > +	totalram_pages += (1UL << order);
>> > > +#ifdef CONFIG_HIGHMEM
>> > > +	if (PageHighMem(page))
>> > > +		totalhigh_pages += (1UL << order);
>> > > +#endif
>> >
>> > __online_page_increment_counters() would have used
>> > adjust_managed_page_count() which would do the changes under
>> > managed_page_count_lock. Are we safe without the lock? If yes, there
>> > should perhaps be a comment explaining why.
>> 
>> Looks unsafe without managed_page_count_lock.
> 
> Why does it matter actually? We cannot online/offline memory in
> parallel. This is not the case for the boot where we initialize memory
> in parallel on multiple nodes. So this seems to be safe currently 
> unless
> I am missing something. A comment explaining that would be helpful
> though.

Other main callers of adjust_manage_page_count(),

static inline void free_reserved_page(struct page *page)
{
         __free_reserved_page(page);
         adjust_managed_page_count(page, 1);
}

static inline void mark_page_reserved(struct page *page)
{
         SetPageReserved(page);
         adjust_managed_page_count(page, -1);
}

Won't they race with memory hotplug?

Few more,
./drivers/xen/balloon.c:519:            adjust_managed_page_count(page, 
-1);
./drivers/virtio/virtio_balloon.c:175:  adjust_managed_page_count(page, 
-1);
./drivers/virtio/virtio_balloon.c:196:  adjust_managed_page_count(page, 
1);
./mm/hugetlb.c:2158:                    adjust_managed_page_count(page, 
1 << h->order);

Regards,
Arun