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

From: Juergen Gross
Date: Thu Sep 27 2018 - 03:11:14 EST


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@xxxxxxxxxxxxxx>
> ---
> 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