Date:   Tue, 20 Sep 2022 13:53:33 +0800
From:   Hongchen Zhang <zhanghongchen@...ngson.cn>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Feng Tang <feng.tang@...el.com>
Subject: Re: Re: [PATCH] mm/vmscan: don't scan adjust too much if current is
 not kswapd

Hi Yosry,

Sorry for not replying in time, there was a problem with my email.

On 2022/9/20 am 7:32, Yosry Ahmed wrote:
> On Fri, Sep 16, 2022 at 3:20 AM Hongchen Zhang
> <zhanghongchen@...ngson.cn> wrote:
>>
>> Hi Andrew and Matthew,
>>
>> On 2022/9/16 pm 4:40, Matthew Wilcox wrote:
>>> On Fri, Sep 16, 2022 at 08:57:50AM +0800, Hongchen Zhang wrote:
>>>> Hi Andrew ,
>>>>
>>>> On 2022/9/15 pm 5:00, Matthew Wilcox wrote:
>>>>> On Thu, Sep 15, 2022 at 04:02:41PM +0800, Hongchen Zhang wrote:
>>>>>> Hi Matthew,
>>>>>> On 2022/9/15 pm 3:28, Matthew Wilcox wrote:
>>>>>>> On Thu, Sep 15, 2022 at 09:19:48AM +0800, Hongchen Zhang wrote:
>>>>>>>> [ 3748.453561] INFO: task float_bessel:77920 blocked for more than 120
>>>>>>>> seconds.
>>>>>>>> [ 3748.460839]       Not tainted 5.15.0-46-generic #49-Ubuntu
>>>>>>>> [ 3748.466490] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables
>>>>>>>> this message.
>>>>>>>> [ 3748.474618] task:float_bessel    state:D stack:    0 pid:77920 ppid:
>>>>>>>> 77327 flags:0x00004002
>>>>>>>> [ 3748.483358] Call Trace:
>>>>>>>> [ 3748.485964]  <TASK>
>>>>>>>> [ 3748.488150]  __schedule+0x23d/0x590
>>>>>>>> [ 3748.491804]  schedule+0x4e/0xc0
>>>>>>>> [ 3748.495038]  rwsem_down_read_slowpath+0x336/0x390
>>>>>>>> [ 3748.499886]  ? copy_user_enhanced_fast_string+0xe/0x40
>>>>>>>> [ 3748.505181]  down_read+0x43/0xa0
>>>>>>>> [ 3748.508518]  do_user_addr_fault+0x41c/0x670
>>>>>>>> [ 3748.512799]  exc_page_fault+0x77/0x170
>>>>>>>> [ 3748.516673]  asm_exc_page_fault+0x26/0x30
>>>>>>>> [ 3748.520824] RIP: 0010:copy_user_enhanced_fast_string+0xe/0x40
>>>>>>>> [ 3748.526764] Code: 89 d1 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 31 c0 0f
>>>>>>>> 01 ca c3 cc cc cc cc 0f 1f 00 0f 01 cb 83 fa 40 0f 82 70 ff ff ff 89 d1 <f3>
>>>>>>>> a4 31 c0 0f 01 ca c3 cc cc cc cc 66 08
>>>>>>>> [ 3748.546120] RSP: 0018:ffffaa9248fffb90 EFLAGS: 00050206
>>>>>>>> [ 3748.551495] RAX: 00007f99faa1a010 RBX: ffffaa9248fffd88 RCX:
>>>>>>>> 0000000000000010
>>>>>>>> [ 3748.558828] RDX: 0000000000001000 RSI: ffff9db397ab8ff0 RDI:
>>>>>>>> 00007f99faa1a000
>>>>>>>> [ 3748.566160] RBP: ffffaa9248fffbf0 R08: ffffcc2fc2965d80 R09:
>>>>>>>> 0000000000000014
>>>>>>>> [ 3748.573492] R10: 0000000000000000 R11: 0000000000000014 R12:
>>>>>>>> 0000000000001000
>>>>>>>> [ 3748.580858] R13: 0000000000001000 R14: 0000000000000000 R15:
>>>>>>>> ffffaa9248fffd98
>>>>>>>> [ 3748.588196]  ? copy_page_to_iter+0x10e/0x400
>>>>>>>> [ 3748.592614]  filemap_read+0x174/0x3e0
>>>>>>>
>>>>>>> Interesting; it wasn't the process itself which triggered the page
>>>>>>> fault; the process called read() and the kernel took the page fault to
>>>>>>> satisfy the read() call.
>>>>>>>
>>>>>>>> [ 3748.596354]  ? ima_file_check+0x6a/0xa0
>>>>>>>> [ 3748.600301]  generic_file_read_iter+0xe5/0x150
>>>>>>>> [ 3748.604884]  ext4_file_read_iter+0x5b/0x190
>>>>>>>> [ 3748.609164]  ? aa_file_perm+0x102/0x250
>>>>>>>> [ 3748.613125]  new_sync_read+0x10d/0x1a0
>>>>>>>> [ 3748.617009]  vfs_read+0x103/0x1a0
>>>>>>>> [ 3748.620423]  ksys_read+0x67/0xf0
>>>>>>>> [ 3748.623743]  __x64_sys_read+0x19/0x20
>>>>>>>> [ 3748.627511]  do_syscall_64+0x59/0xc0
>>>>>>>> [ 3748.631203]  ? syscall_exit_to_user_mode+0x27/0x50
>>>>>>>> [ 3748.636144]  ? do_syscall_64+0x69/0xc0
>>>>>>>> [ 3748.639992]  ? exit_to_user_mode_prepare+0x96/0xb0
>>>>>>>> [ 3748.644931]  ? irqentry_exit_to_user_mode+0x9/0x20
>>>>>>>> [ 3748.649872]  ? irqentry_exit+0x1d/0x30
>>>>>>>> [ 3748.653737]  ? exc_page_fault+0x89/0x170
>>>>>>>> [ 3748.657795]  entry_SYSCALL_64_after_hwframe+0x61/0xcb
>>>>>>>> [ 3748.663030] RIP: 0033:0x7f9a852989cc
>>>>>>>> [ 3748.666713] RSP: 002b:00007f9a8497dc90 EFLAGS: 00000246 ORIG_RAX:
>>>>>>>> 0000000000000000
>>>>>>>> [ 3748.674487] RAX: ffffffffffffffda RBX: 00007f9a8497f5c0 RCX:
>>>>>>>> 00007f9a852989cc
>>>>>>>> [ 3748.681817] RDX: 0000000000027100 RSI: 00007f99faa18010 RDI:
>>>>>>>> 0000000000000061
>>>>>>>> [ 3748.689150] RBP: 00007f9a8497dd60 R08: 0000000000000000 R09:
>>>>>>>> 00007f99faa18010
>>>>>>>> [ 3748.696493] R10: 0000000000000000 R11: 0000000000000246 R12:
>>>>>>>> 00007f99faa18010
>>>>>>>> [ 3748.703841] R13: 00005605e11c406f R14: 0000000000000001 R15:
>>>>>>>> 0000000000027100
>>>>>>>
>>>>>>> ORIG_RAX is 0, which matches sys_read.
>>>>>>> RDI is file descriptor 0x61
>>>>>>> RSI is plausibly a userspace pointer, 0x7f99faa18010
>>>>>>> RDX is the length, 0x27100 or 160kB.
>>>>>>>
>>>>>>> That all seems reasonable.
>>>>>>>
>>>>>>> What I really want to know is who is _holding_ the lock.  We stash
>>>>>>> a pointer to the task_struct in 'owner', so we could clearly find this
>>>>>>> out in the 'blocked for too long' report, and print their stack trace.
>>>>>>>
>>>>>> As described in the comment for __rwsem_set_reader_owned,it is hard to track
>>>>>> read owners.So we could not clearly find out who blocked the process,it was
>>>>>> caused by multiple tasks.
>>>>>
>>>>> Readers don't block readers.  You have a reader here, so it's being
>>>>> blocked by a writer.  And that writer's task_struct is stashed in
>>>>> rwsem->owner.  It would be nice if we dumped that information
>>>>> automatically ... but we don't do that today.  Perhaps you could
>>>>> grab that information from a crash dump if you have one.
>>>>>
>>>>>>> You must have done something like this already in order to deduce that
>>>>>>> it was the direct reclaim path that was the problem?
>>>>>>>
>>>>>> The method we used is to track the direct reclaim using the
>>>>>> trace_mm_vmscan_direct_reclaim_{begin,end} interface.When the problem
>>>>>> occurred,we could get a very large "nr_reclaimed" which is not a desirable
>>>>>> value for process except kswapd.
>>>>>
>>>>> I disagree.  If a process needs to allocate memory then it should be
>>>>> paying the cost of reclaiming that memory itself.  kswapd is a last
>>>>> resort to reclaim memory when we have a workload (eg a network router)
>>>>> that does its memory allocation primarily in interrupt context.
>>>>>
>>>> What's your opinion about this scan adjust issue? Is there a better way to
>>>> fix this issue?
>>>
>>> Yes, but we need you to gather more information about what's causing
>>> the issue before we can suggest what that is.
>>>
>> I think the following scenery triggers the scan adjust issue:
>> In function shrink_lruvec, we call get_scan_count and get the following
>> values:
>> targets[LRU_INACTIVE_ANON]=50000
>> targets[LRU_ACTIVE_ANON]=50000
>> targets[LRU_INACTIVE_FILE]=128
>> targets[LRU_ACTIVE_FILE]=129
>>
>> After the first scan, we get more than nr_to_reclaim pages, but the
>> percentage of scanning nr[LRU_INACTIVE_FILE+LRU_ACTIVE_FILE] is 256/257,
>>
>> Then when we scan adjust, we must scan(possibly reclaim)
>> 256*(50000+50000)/257-256=99354 pages, which is too large and would
>> waste too many time.
>> If it is not kswapd, it is unacceptable to reclaim so many pages.
>> So we should limit the number of pages of scan adjust.
> 
> IIUC commit 6eb90d649537 ("mm: vmscan: fix extreme overreclaim and
> swap floods") that was recently sent by Johannes [1] addresses a
> similar issue (reclaiming way beyond nr_to_reclaim when anon vs file
> LRU sizes are very different), but in a slightly different scenario.
> IIUC with Johannes's patch, scan adjustment is already limited for
> scenarios where scan_adjust (aka proportional_reclaim) is not
> initialized to true, which would be all cases except global direct
> reclaim on DEF_PRIORITY. Is my understanding here correct?
> 
Yes, this patch fix the same issue,let's talk this issue there.
> [1] https://lore.kernel.org/lkml/20220802162811.39216-1-hannes@cmpxchg.org/
>>
>> Thanks
>> Hongchen Zhang
>>
>>
> 
> 

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