From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: Barry Song <21cnbao@gmail.com>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org, david@redhat.com,
shikemeng@huaweicloud.com, willy@infradead.org,
mgorman@techsingularity.net, hannes@cmpxchg.org,
linux-kernel@vger.kernel.org, Barry Song <v-songbaohua@oppo.com>,
Zhanyuan Hu <huzhanyuan@oppo.com>
Subject: Re: [RFC PATCH] mm: compaction: avoid fast_isolate_freepages blindly choose improper pageblock
Date: Thu, 7 Dec 2023 09:50:43 +0800 [thread overview]
Message-ID: <1ba8b967-8f35-4144-8b7c-836b288ca8d6@linux.alibaba.com> (raw)
In-Reply-To: <CAGsJ_4y4Me8=sgujeg6zSz2c_LmnSEg=Z+Q1C7_Wug=k0bGaug@mail.gmail.com>
On 12/6/2023 6:18 PM, Barry Song wrote:
> On Wed, Dec 6, 2023 at 10:54 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 11/29/2023 6:45 PM, Barry Song wrote:
>>> Testing shows fast_isolate_freepages can blindly choose an unsuitable
>>> pageblock from time to time particularly while the min mark is used
>>> from XXX path:
>>> if (!page) {
>>> cc->fast_search_fail++;
>>> if (scan_start) {
>>> /*
>>> * Use the highest PFN found above min. If one was
>>> * not found, be pessimistic for direct compaction
>>> * and use the min mark.
>>> */
>>> if (highest >= min_pfn) {
>>> page = pfn_to_page(highest);
>>> cc->free_pfn = highest;
>>> } else {
>>> if (cc->direct_compaction && pfn_valid(min_pfn)) { /* XXX */
>>> page = pageblock_pfn_to_page(min_pfn,
>>> min(pageblock_end_pfn(min_pfn),
>>> zone_end_pfn(cc->zone)),
>>> cc->zone);
>>> cc->free_pfn = min_pfn;
>>> }
>>> }
>>> }
>>> }
>>
>> Yes, the min_pfn can be an unsuitable migration target. But I think we
>> can just add the suitable_migration_target() validation into 'min_pfn'
>> case? Since other cases must be suitable target which found from
>> MIGRATE_MOVABLE free list. Something like below:
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 01ba298739dd..4e8eb4571909 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1611,6 +1611,8 @@ static void fast_isolate_freepages(struct
>> compact_control *cc)
>>
>> min(pageblock_end_pfn(min_pfn),
>>
>> zone_end_pfn(cc->zone)),
>> cc->zone);
>> + if
>> (!suitable_migration_target(cc, page))
>> + page = NULL;
>> cc->free_pfn = min_pfn;
>> }
>> }
>>
>
> yes. this makes more senses.
>
>> By the way, I wonder if this patch can improve the efficiency of
>> compaction in your test case?
>
> This happens not quite often. when running 25 machines for
> one night, most of them can hit this unexpected code path.
> but the frequency isn't many times in one second. it might
> be one time in a couple of hours.
>
> so it is very difficult to measure the visible performance impact
> in my machines though the affection of choosing the unsuitable
> migration_target should be negative.
OK. Fair enough.
>
> I feel like it's worth fixing this to at least make the code theoretically
> self-explanatory? as it is quite odd unsuitable_migration_target can
> be still migration_target?
>
>>
>>> In contrast, slow path is skipping unsuitable pageblocks in a decent way.
>>>
>>> I don't know if it is an intended design or just an oversight. But
>>> it seems more sensible to skip unsuitable pageblock.
>>>
>>> Reported-by: Zhanyuan Hu <huzhanyuan@oppo.com>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>> ---
>>> mm/compaction.c | 6 ++++++
>>> 1 file changed, 6 insertions(+)
>>>
>>> diff --git a/mm/compaction.c b/mm/compaction.c
>>> index 01ba298739dd..98c485a25614 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -1625,6 +1625,12 @@ static void fast_isolate_freepages(struct compact_control *cc)
>>> cc->total_free_scanned += nr_scanned;
>>> if (!page)
>>> return;
>>> + /*
>>> + * Otherwise, we can blindly choose an improper pageblock especially
>>> + * while using the min mark
>>> + */
>>> + if (!suitable_migration_target(cc, page))
>>> + return;
>>>
>>> low_pfn = page_to_pfn(page);
>>> fast_isolate_around(cc, low_pfn);
>
> Thanks
> Barry
prev parent reply other threads:[~2023-12-07 1:50 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-29 10:45 [RFC PATCH] mm: compaction: avoid fast_isolate_freepages blindly choose improper pageblock Barry Song
2023-12-06 9:54 ` Baolin Wang
2023-12-06 10:18 ` Barry Song
2023-12-07 1:50 ` Baolin Wang [this message]
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