Re: [PATCH] memory_hotplug: Free pages as higher order

[Arun KS <arunks@codeaurora.org>]

On 2018-09-21 21:12, Dan Williams wrote:
> On Fri, Sep 21, 2018 at 2:40 AM Arun KS <arunks@codeaurora.org> 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@codeaurora.org>
>> 
>> ---
>> 
>> Changes since RFC:
>> - Rebase.
>> - As suggested by Michal Hocko remove pages_per_block.
>> - Modifed external providers of online_page_callback.
>> 
>> RFC:
>> https://lore.kernel.org/patchwork/patch/984754/
>> ---
>>  drivers/hv/hv_balloon.c        |  6 +++--
>>  drivers/xen/balloon.c          | 18 +++++++++++---
>>  include/linux/memory_hotplug.h |  2 +-
>>  mm/memory_hotplug.c            | 55 
>> +++++++++++++++++++++++++++++++++---------
>>  4 files changed, 63 insertions(+), 18 deletions(-)
>> 
>> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
>> index b1b7880..c5bc0b5 100644
>> --- a/drivers/hv/hv_balloon.c
>> +++ b/drivers/hv/hv_balloon.c
>> @@ -771,7 +771,7 @@ static void hv_mem_hot_add(unsigned long start, 
>> unsigned long size,
>>         }
>>  }
>> 
>> -static void hv_online_page(struct page *pg)
>> +static int hv_online_page(struct page *pg, unsigned int order)
>>  {
>>         struct hv_hotadd_state *has;
>>         unsigned long flags;
>> @@ -783,10 +783,12 @@ static void hv_online_page(struct page *pg)
>>                 if ((pfn < has->start_pfn) || (pfn >= has->end_pfn))
>>                         continue;
>> 
>> -               hv_page_online_one(has, pg);
>> +               hv_bring_pgs_online(has, pfn, (1UL << order));
>>                 break;
>>         }
>>         spin_unlock_irqrestore(&dm_device.ha_lock, flags);
>> +
>> +       return 0;
>>  }
>> 
>>  static int pfn_covered(unsigned long start_pfn, unsigned long 
>> pfn_cnt)
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index e12bb25..010cf4d 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -390,8 +390,8 @@ static enum bp_state 
>> reserve_additional_memory(void)
>> 
>>         /*
>>          * add_memory_resource() will call online_pages() which in its 
>> turn
>> -        * will call xen_online_page() callback causing deadlock if we 
>> don't
>> -        * release balloon_mutex here. Unlocking here is safe because 
>> the
>> +        * will call xen_bring_pgs_online() callback causing deadlock 
>> if we
>> +        * don't release balloon_mutex here. Unlocking here is safe 
>> because the
>>          * callers drop the mutex before trying again.
>>          */
>>         mutex_unlock(&balloon_mutex);
>> @@ -422,6 +422,18 @@ static void xen_online_page(struct page *page)
>>         mutex_unlock(&balloon_mutex);
>>  }
>> 
>> +static int xen_bring_pgs_online(struct page *pg, unsigned int order)
>> +{
>> +       unsigned long i, size = (1 << order);
>> +       unsigned long start_pfn = page_to_pfn(pg);
>> +
>> +       pr_debug("Online %lu pages starting at pfn 0x%lx\n", size, 
>> start_pfn);
>> +       for (i = 0; i < size; i++)
>> +               xen_online_page(pfn_to_page(start_pfn + i));
>> +
>> +       return 0;
>> +}
>> +
>>  static int xen_memory_notifier(struct notifier_block *nb, unsigned 
>> long val, void *v)
>>  {
>>         if (val == MEM_ONLINE)
>> @@ -744,7 +756,7 @@ static int __init balloon_init(void)
>>         balloon_stats.max_retry_count = RETRY_UNLIMITED;
>> 
>>  #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
>> -       set_online_page_callback(&xen_online_page);
>> +       set_online_page_callback(&xen_bring_pgs_online);
>>         register_memory_notifier(&xen_memory_nb);
>>         register_sysctl_table(xen_root);
>> 
>> diff --git a/include/linux/memory_hotplug.h 
>> b/include/linux/memory_hotplug.h
>> index 34a2822..7b04c1d 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -87,7 +87,7 @@ extern int test_pages_in_a_zone(unsigned long 
>> start_pfn, unsigned long end_pfn,
>>         unsigned long *valid_start, unsigned long *valid_end);
>>  extern void __offline_isolated_pages(unsigned long, unsigned long);
>> 
>> -typedef void (*online_page_callback_t)(struct page *page);
>> +typedef int (*online_page_callback_t)(struct page *page, unsigned int 
>> order);
>> 
>>  extern int set_online_page_callback(online_page_callback_t callback);
>>  extern int restore_online_page_callback(online_page_callback_t 
>> callback);
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 38d94b7..24c2b8e 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -47,7 +47,7 @@
>>   * and restore_online_page_callback() for generic callback restore.
>>   */
>> 
>> -static void generic_online_page(struct page *page);
>> +static int generic_online_page(struct page *page, unsigned int 
>> order);
>> 
>>  static online_page_callback_t online_page_callback = 
>> generic_online_page;
>>  static DEFINE_MUTEX(online_page_callback_lock);
>> @@ -655,26 +655,57 @@ 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);
>> -       __online_page_increment_counters(page);
>> -       __online_page_free(page);
>> +       unsigned long nr_pages = 1 << order;
>> +       struct page *p = page;
>> +       unsigned int loop;
>> +
>> +       prefetchw(p);
>> +       for (loop = 0 ; loop < (nr_pages - 1) ; loop++, p++) {
>> +               prefetch(p + 1);
> 
> Given commits like:
> 
> e66eed651fd1 list: remove prefetching from regular list iterators
> 75d65a425c01 hlist: remove software prefetching in hlist iterators
> 
> ...are you sure these explicit prefetch() calls are improving
> performance? My understanding is that hardware prefetchers don't need
> much help these days.
Hello Dan,

Thanks for your comment. I tested on arm64 with and without prefetch and 
as you guessed, the one without prefetch is slightly better.

Will remove prefetch before sending next version.

Regards,
Arun