linux-kernel - Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about stalls

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Date:   Wed, 13 Sep 2017 16:03:51 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        mhocko@...nel.org, mpatocka@...hat.com
Cc:     hannes@...xchg.org, mgorman@...e.de, dave.hansen@...el.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about
 stalls

On 09/13/2017 03:54 PM, Tetsuo Handa wrote:
> Michal Hocko wrote:
>> On Mon 11-09-17 19:36:59, Mikulas Patocka wrote:
>>>
>>>
>>> On Mon, 11 Sep 2017, Michal Hocko wrote:
>>>
>>>> On Mon 11-09-17 02:52:53, Mikulas Patocka wrote:
>>>>> I am occasionally getting these warnings in khugepaged. It is an old 
>>>>> machine with 550MHz CPU and 512 MB RAM.
>>>>>
>>>>> Note that khugepaged has nice value 19, so when the machine is loaded with 
>>>>> some work, khugepaged is stalled and this stall produces warning in the 
>>>>> allocator.
>>>>>
>>>>> khugepaged does allocations with __GFP_NOWARN, but the flag __GFP_NOWARN
>>>>> is masked off when calling warn_alloc. This patch removes the masking of
>>>>> __GFP_NOWARN, so that the warning is suppressed.
>>>>>
>>>>> khugepaged: page allocation stalls for 10273ms, order:10, mode:0x4340ca(__GFP_HIGHMEM|__GFP_IO|__GFP_FS|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL|__GFP_MOVABLE|__GFP_DIRECT_RECLAIM), nodemask=(null)
>>>>> CPU: 0 PID: 3936 Comm: khugepaged Not tainted 4.12.3 #1
>>>>> Hardware name: System Manufacturer Product Name/VA-503A, BIOS 4.51 PG 08/02/00
>>>>> Call Trace:
>>>>>  ? warn_alloc+0xb9/0x140
>>>>>  ? __alloc_pages_nodemask+0x724/0x880
>>>>>  ? arch_irq_stat_cpu+0x1/0x40
>>>>>  ? detach_if_pending+0x80/0x80
>>>>>  ? khugepaged+0x10a/0x1d40
>>>>>  ? pick_next_task_fair+0xd2/0x180
>>>>>  ? wait_woken+0x60/0x60
>>>>>  ? kthread+0xcf/0x100
>>>>>  ? release_pte_page+0x40/0x40
>>>>>  ? kthread_create_on_node+0x40/0x40
>>>>>  ? ret_from_fork+0x19/0x30
>>>>>
>>>>> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
>>>>> Cc: stable@...r.kernel.org
>>>>> Fixes: 63f53dea0c98 ("mm: warn about allocations which stall for too long")
>>>>
>>>> This patch hasn't introduced this behavior. It deliberately skipped
>>>> warning on __GFP_NOWARN. This has been introduced later by 822519634142
>>>> ("mm: page_alloc: __GFP_NOWARN shouldn't suppress stall warnings"). I
>>>> disagreed [1] but overall consensus was that such a warning won't be
>>>> harmful. Could you be more specific why do you consider it wrong,
>>>> please?
>>>
>>> I consider the warning wrong, because it warns when nothing goes wrong. 
>>> I've got 7 these warnings for 4 weeks of uptime. The warnings typically 
>>> happen when I run some compilation.
>>>
>>> A process with low priority is expected to be running slowly when there's 
>>> some high-priority process, so there's no need to warn that the 
>>> low-priority process runs slowly.
>>
>> I would tend to agree. It is certainly a noise in the log. And a kind of
>> thing I was worried about when objecting the patch previously. 
>>  
>>> What else can be done to avoid the warning? Skip the warning if the 
>>> process has lower priority?
>>
>> No, I wouldn't play with priorities. Either we agree that NOWARN
>> allocations simply do _not_warn_ or we simply explain users that some of
>> those warnings might not be that critical and overloaded system might
>> show them.
>>
>> Let's see what others think about this.
> 
> Whether __GFP_NOWARN should warn about stalls is not a topic to discuss.

It is the topic of this thread, which tries to address a concrete
problem somebody has experienced. In that context, the rest of your
concerns seem to me not related to this problem, IMHO.

> I consider warn_alloc() for reporting stalls is broken. It fails to provide
> backtrace of stalling location. For example, OOM lockup with oom_lock held
> cannot be reported by warn_alloc(). It fails to provide readable output when
> called concurrently. For example, concurrent calls can cause printk()/
> schedule_timeout_killable() lockup with oom_lock held. printk() offloading is
> not an option, for there will be situations where printk() offloading cannot
> be used (e.g. queuing via printk() is faster than writing to serial consoles
> which results in unreadable logs due to log_bug overflow).
>