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Message-ID: <320c16a7-96b7-65ec-3d80-2eace0ddb290@suse.cz>
Date: Wed, 13 Sep 2023 11:33:52 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mgorman@...hsingularity.net>,
Miaohe Lin <linmiaohe@...wei.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm: page_alloc: remove pcppage migratetype caching
On 9/12/23 16:50, Johannes Weiner wrote:
> On Tue, Sep 12, 2023 at 03:47:45PM +0200, Vlastimil Babka wrote:
>> On 9/11/23 21:41, Johannes Weiner wrote:
>
>> > @@ -1577,7 +1556,6 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>> > continue;
>> > del_page_from_free_list(page, zone, current_order);
>> > expand(zone, page, order, current_order, migratetype);
>> > - set_pcppage_migratetype(page, migratetype);
>>
>> Hm interesting, just noticed that __rmqueue_fallback() never did this
>> AFAICS, sounds like a bug.
>
> I don't quite follow. Which part?
>
> Keep in mind that at this point __rmqueue_fallback() doesn't return a
> page. It just moves pages to the desired freelist, and then
> __rmqueue_smallest() gets called again. This changes in 5/6, but until
> now at least all of the above would apply to fallback pages.
Yep, missed that "doesn't return a page", thanks.
>> > @@ -2145,7 +2123,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>> > * pages are ordered properly.
>> > */
>> > list_add_tail(&page->pcp_list, list);
>> > - if (is_migrate_cma(get_pcppage_migratetype(page)))
>> > + if (is_migrate_cma(get_pageblock_migratetype(page)))
>> > __mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
>> > -(1 << order));
>>
>> This is potentially a source of overhead, I assume patch 6/6 might
>> change that.
>
> Yes, 6/6 removes it altogether.
>
> But the test results in this patch's changelog are from this patch in
> isolation, so it doesn't appear to be a concern even on its own.
>
>> > @@ -2457,7 +2423,7 @@ void free_unref_page_list(struct list_head *list)
>> > * Free isolated pages directly to the allocator, see
>> > * comment in free_unref_page.
>> > */
>> > - migratetype = get_pcppage_migratetype(page);
>> > + migratetype = get_pfnblock_migratetype(page, pfn);
>> > if (unlikely(is_migrate_isolate(migratetype))) {
>> > list_del(&page->lru);
>> > free_one_page(page_zone(page), page, pfn, 0, migratetype, FPI_NONE);
>>
>> I think after this change we should move the isolated pages handling to
>> the second loop below, so that we wouldn't have to call
>> get_pfnblock_migratetype() twice per page. Dunno yet if some later patch
>> does that. It would need to unlock pcp when necessary.
>
> That sounds like a great idea. Something like the following?
>
> Lightly tested. If you're good with it, I'll beat some more on it and
> submit it as a follow-up.
>
> ---
>
> From 429d13322819ab38b3ba2fad6d1495997819ccc2 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Tue, 12 Sep 2023 10:16:10 -0400
> Subject: [PATCH] mm: page_alloc: optimize free_unref_page_list()
>
> Move direct freeing of isolated pages to the lock-breaking block in
> the second loop. This saves an unnecessary migratetype reassessment.
>
> Minor comment and local variable scoping cleanups.
Looks like batch_count and locked_zone could be moved to the loop scope as well.
>
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> mm/page_alloc.c | 49 +++++++++++++++++++++----------------------------
> 1 file changed, 21 insertions(+), 28 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e3f1c777feed..9cad31de1bf5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2408,48 +2408,41 @@ void free_unref_page_list(struct list_head *list)
> struct per_cpu_pages *pcp = NULL;
> struct zone *locked_zone = NULL;
> int batch_count = 0;
> - int migratetype;
> -
> - /* Prepare pages for freeing */
> - list_for_each_entry_safe(page, next, list, lru) {
> - unsigned long pfn = page_to_pfn(page);
>
> - if (!free_pages_prepare(page, 0, FPI_NONE)) {
> + list_for_each_entry_safe(page, next, list, lru)
> + if (!free_pages_prepare(page, 0, FPI_NONE))
> list_del(&page->lru);
> - continue;
> - }
> -
> - /*
> - * Free isolated pages directly to the allocator, see
> - * comment in free_unref_page.
> - */
> - migratetype = get_pfnblock_migratetype(page, pfn);
> - if (unlikely(is_migrate_isolate(migratetype))) {
> - list_del(&page->lru);
> - free_one_page(page_zone(page), page, pfn, 0, migratetype, FPI_NONE);
> - continue;
> - }
> - }
>
> list_for_each_entry_safe(page, next, list, lru) {
> unsigned long pfn = page_to_pfn(page);
> struct zone *zone = page_zone(page);
> + int migratetype;
>
> list_del(&page->lru);
> migratetype = get_pfnblock_migratetype(page, pfn);
>
> /*
> - * Either different zone requiring a different pcp lock or
> - * excessive lock hold times when freeing a large list of
> - * pages.
> + * Zone switch, batch complete, or non-pcp freeing?
> + * Drop the pcp lock and evaluate.
> */
> - if (zone != locked_zone || batch_count == SWAP_CLUSTER_MAX) {
> + if (unlikely(zone != locked_zone ||
> + batch_count == SWAP_CLUSTER_MAX ||
> + is_migrate_isolate(migratetype))) {
> if (pcp) {
> pcp_spin_unlock(pcp);
> pcp_trylock_finish(UP_flags);
> + locked_zone = NULL;
> }
>
> - batch_count = 0;
> + /*
> + * Free isolated pages directly to the
> + * allocator, see comment in free_unref_page.
> + */
> + if (is_migrate_isolate(migratetype)) {
> + free_one_page(zone, page, pfn, 0,
> + migratetype, FPI_NONE);
> + continue;
> + }
>
> /*
> * trylock is necessary as pages may be getting freed
> @@ -2459,12 +2452,12 @@ void free_unref_page_list(struct list_head *list)
> pcp = pcp_spin_trylock(zone->per_cpu_pageset);
> if (unlikely(!pcp)) {
> pcp_trylock_finish(UP_flags);
> - free_one_page(zone, page, pfn,
> - 0, migratetype, FPI_NONE);
> - locked_zone = NULL;
> + free_one_page(zone, page, pfn, 0,
> + migratetype, FPI_NONE);
> continue;
> }
> locked_zone = zone;
> + batch_count = 0;
> }
>
> /*
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