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Subject: Re: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags
To:     David Hildenbrand <david@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <1597549677-7480-1-git-send-email-alex.shi@linux.alibaba.com>
 <eeb80957-96f7-661c-1510-0e6247570bd3@redhat.com>
From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <89b42892-8257-e3fc-34c9-27d9a470ed79@linux.alibaba.com>
Date:   Sun, 16 Aug 2020 22:14:08 +0800
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在 2020/8/16 下午8:16, David Hildenbrand 写道:
> On 16.08.20 05:47, Alex Shi wrote:
>> pageblock_flags is used as long, since every pageblock_flags is just 4
>> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags,
>> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock
>> flags. It would cause long waiting for sync.
>>
>> If we could change the pageblock_flags variable as char, we could use
>> char size cmpxchg, which just sync up with 2 pageblock flags. it could
>> relief much false sharing in cmpxchg.
> 
> Do you have any performance numbers to back your claims? IOW, do we care
> at all?
> 
> 

Hi David,

Thanks for comments!

Not yet, I trace the following commit here, with this commit this hot path
looks be resolved.

Thanks
Alex

e380bebe4771548  mm, compaction: keep migration source private to a single compaction instance
                if (!locked) {
                        locked = compact_trylock_irqsave(zone_lru_lock(zone),
                                                                &flags, cc);
-                       if (!locked)
+
+                       /* Allow future scanning if the lock is contended */
+                       if (!locked) {
+                               clear_pageblock_skip(page);
                                break;
+                       }
+
+                       /* Try get exclusive access under lock */
+                       if (!skip_updated) {
+                               skip_updated = true;
+                               if (test_and_set_skip(cc, page, low_pfn))
+                                       goto isolate_abort;
+                       }