Re: [PATCH v3] memory_hotplug: Free pages as higher order

[Juergen Gross <jgross@suse.com>]

On 27/09/18 08:58, 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@codeaurora.org>
> ---
> Changes since v2:
> reuse code from __free_pages_boot_core()
> 
> Changes since v1:
> - Removed prefetch()
> 
> Changes since RFC:
> - Rebase.
> - As suggested by Michal Hocko remove pages_per_block.
> - Modifed external providers of online_page_callback.
> 
> v2: https://lore.kernel.org/patchwork/patch/991363/
> v1: https://lore.kernel.org/patchwork/patch/989445/
> 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/internal.h                  |  1 +
>  mm/memory_hotplug.c            | 44 ++++++++++++++++++++++++++++++------------
>  mm/page_alloc.c                |  2 +-
>  6 files changed, 54 insertions(+), 19 deletions(-)
>

...

> 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));

xen_online_page() isn't very complex and this is the only user.

Why don't you move its body in here and drop the extra function?
And now you can execute the loop with balloon_mutex held instead of
taking and releasing it in each iteration of the loop.


Juergen