Re: [RFC] memory_hotplug: Free pages as pageblock_order

[Arun KS]

On 2018-09-12 18:47, Michal Hocko wrote:
> On Wed 12-09-18 22:57:43, Balbir Singh wrote:
> > On Wed, Sep 12, 2018 at 12:38:53PM +0200, Michal Hocko wrote:
> > > On Wed 12-09-18 14:56:45, Arun KS wrote:
> > > > When free pages are done with pageblock_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%.
> > >
> > > Where does the improvement come from? You are still doing the same
> > > amount of work except that the number of callbacks is lower. Is this the
> > > real source of 60% improvement?
> > >
> >
> > It looks like only the first page of the pageblock is initialized, is
> > some of the cost amortized in terms of doing one initialization for
> > the page with order (order) and then relying on split_page and helpers
> > to do the rest? Of course the number of callbacks reduce by a 
> > significant
> > number as well.
>
> Ohh, I have missed that part. Now when re-reading I can see the reason
> for the perf improvement. It is most likely the higher order free which
> ends up being much cheaper. This part makes some sense.
>
> How much is this feasible is another question. Do not forget we have
> those external providers of the online callback and those would need to
> be updated as well.
Sure Michal, I ll look into this.

> Btw. the normal memmap init code path does the same per-page free as
> well. If we really want to speed the hotplug path then I guess the init
> one would see a bigger improvement and those two should be in sync.
Thanks for pointers, Will look further.

> > > >
> > > > If this looks okey, I'll modify users of set_online_page_callback
> > > > and resend clean patch.
> > >
> > > [...]
> > >
> > > > +static int generic_online_pages(struct page *page, unsigned int order);
> > > > +static online_pages_callback_t online_pages_callback = generic_online_pages;
> > > > +
> > > > +static int generic_online_pages(struct page *page, unsigned int order)
> > > > +{
> > > > +	unsigned long nr_pages = 1 << order;
> > > > +	struct page *p = page;
> > > > +	unsigned int loop;
> > > > +
> > > > +	for (loop = 0 ; loop < nr_pages ; loop++, p++) {
> > > > +		__ClearPageReserved(p);
> > > > +		set_page_count(p, 0);
>
> btw. you want init_page_count here.
Do you mean replace set_page_count(p, 0) with init_page_count(page)?
Because init_page_count is setting the page _refcount to 1

static inline void init_page_count(struct page *page)
{
        set_page_count(page, 1);
}

I thought in case of higher order pages only the first struct page 
should have _refcount to 1 before calling __free_pages(). Please correct 
me if wrong.

Regards,
Arun