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Subject: Re: [PATCH] mm/vmscan: don't scan adjust too much if current is not
 kswapd
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20220914023318.549118-1-zhanghongchen@loongson.cn>
 <20220914155142.bf388515a39fb45bae987231@linux-foundation.org>
 <6bcb4883-03d0-88eb-4c42-84fff0a9a141@loongson.cn>
 <YyLUGnqtZXn4MjJF@casper.infradead.org>
 <54813a74-cc0e-e470-c632-78437a0d0ad4@loongson.cn>
 <YyLpls9/t6LKQefS@casper.infradead.org>
From:   Hongchen Zhang <zhanghongchen@loongson.cn>
Message-ID: <b52b3f49-ebf5-6f63-da1a-f57711c3f97d@loongson.cn>
Date:   Fri, 16 Sep 2022 08:57:50 +0800
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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?

Thanks
Hongchen Zhang