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Date: Mon, 7 Sep 2020 09:32:37 +0200
From: Michal Hocko <mhocko@...e.com>
To: Pavel Tatashin <pasha.tatashin@...een.com>
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
linux-mm@...ck.org, osalvador@...e.de, richard.weiyang@...il.com,
david@...hat.com, vbabka@...e.cz, rientjes@...gle.com
Subject: Re: [PATCH v3 2/2] mm: drain per-cpu pages outside of
isolate_migratepages_range
On Fri 04-09-20 11:14:48, Pavel Tatashin wrote:
> It is expensive to drain per-cpu page lists as a thread is started on each
> CPU, and we are waiting for them to complete.
>
> Currently, we drain on every block of pages that is isolated. Instead lets
> drain once after pages are isolated.
>
> For example, when 2G of memory is hot-removed drain is called 16 times,
> with this change it will be done only one time on average.
I do agree that the current implementation is much less effective than
it could be but I disagree we should be pushing the burden to all
callers as already stated
(http://lkml.kernel.org/r/20200907072608.GE30144@dhcp22.suse.cz).
I believe it should be perfectly fine to start_isolate_page_range would
improve the situation considerably. There are some minor details to sort
out (multizone pfn span which we do not allow but probably should be
enforcing somehow).
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
I believe we should be going an opposite direction and define a more
understandable and usable semantic for start_isolate_page_range. We do
not want callers to scratch their heads to call all caches they might
need to flush.
> ---
> mm/memory_hotplug.c | 15 +--------------
> mm/page_alloc.c | 2 ++
> mm/page_isolation.c | 40 ++++++++++++----------------------------
> 3 files changed, 15 insertions(+), 42 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b11a269e2356..5a2ed1a94555 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1536,6 +1536,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
> }
>
> do {
> + drain_all_pages(zone);
> pfn = start_pfn;
> do {
> if (signal_pending(current)) {
> @@ -1575,20 +1576,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
> /* check again */
> ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> NULL, check_pages_isolated_cb);
> - /*
> - * per-cpu pages are drained in start_isolate_page_range, but if
> - * there are still pages that are not free, make sure that we
> - * drain again, because when we isolated range we might
> - * have raced with another thread that was adding pages to pcp
> - * list.
> - *
> - * Forward progress should be still guaranteed because
> - * pages on the pcp list can only belong to MOVABLE_ZONE
> - * because has_unmovable_pages explicitly checks for
> - * PageBuddy on freed pages on other zones.
> - */
> - if (ret)
> - drain_all_pages(zone);
> } while (ret);
>
> /* Ok, all of our target is isolated.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fab5e97dc9ca..6d6a501a103e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8462,6 +8462,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> if (ret < 0)
> return ret;
>
> + drain_all_pages(cc.zone);
> +
> /*
> * In case of -EBUSY, we'd like to know which page causes problem.
> * So, just fall through. test_pages_isolated() has a tracepoint
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 63a3db10a8c0..8dfa6c6c668d 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -19,8 +19,8 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> {
> struct page *unmovable = NULL;
> struct zone *zone;
> - unsigned long flags;
> - int ret = -EBUSY;
> + unsigned long flags, nr_pages;
> + int ret = -EBUSY, mt;
>
> zone = page_zone(page);
>
> @@ -39,24 +39,18 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> * We just check MOVABLE pages.
> */
> unmovable = has_unmovable_pages(zone, page, migratetype, isol_flags);
> - if (!unmovable) {
> - unsigned long nr_pages;
> - int mt = get_pageblock_migratetype(page);
> -
> - set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> - zone->nr_isolate_pageblock++;
> - nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE,
> - NULL);
> -
> - __mod_zone_freepage_state(zone, -nr_pages, mt);
> - ret = 0;
> - }
> + if (unmovable)
> + goto out;
>
> + mt = get_pageblock_migratetype(page);
> + set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> + zone->nr_isolate_pageblock++;
> + nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE, NULL);
> + __mod_zone_freepage_state(zone, -nr_pages, mt);
> + ret = 0;
> out:
> spin_unlock_irqrestore(&zone->lock, flags);
> - if (!ret) {
> - drain_all_pages(zone);
> - } else {
> + if (ret) {
> WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
>
> if ((isol_flags & REPORT_FAILURE) && unmovable)
> @@ -170,14 +164,6 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
> * pageblocks we may have modified and return -EBUSY to caller. This
> * prevents two threads from simultaneously working on overlapping ranges.
> *
> - * Please note that there is no strong synchronization with the page allocator
> - * either. Pages might be freed while their page blocks are marked ISOLATED.
> - * In some cases pages might still end up on pcp lists and that would allow
> - * for their allocation even when they are in fact isolated already. Depending
> - * on how strong of a guarantee the caller needs drain_all_pages might be needed
> - * (e.g. __offline_pages will need to call it after check for isolated range for
> - * a next retry).
> - *
> * Return: the number of isolated pageblocks on success and -EBUSY if any part
> * of range cannot be isolated.
> */
> @@ -192,9 +178,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
> BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
> BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
>
> - for (pfn = start_pfn;
> - pfn < end_pfn;
> - pfn += pageblock_nr_pages) {
> + for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
> page = __first_valid_page(pfn, pageblock_nr_pages);
> if (page) {
> if (set_migratetype_isolate(page, migratetype, flags)) {
> --
> 2.25.1
>
--
Michal Hocko
SUSE Labs
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