netdev - Re: [PATCH V4] mlx4_core: allocate ICM memory in page size chunks

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Date:   Tue, 5 Jun 2018 11:51:18 -0700
From:   Qing Huang <qing.huang@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>, tariqt@...lanox.com,
        haakon.bugge@...cle.com, yanjun.zhu@...cle.com,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, gi-oh.kim@...fitbricks.com,
        "santosh.shilimkar@...cle.com" <santosh.shilimkar@...cle.com>,
        rama nichanamatlu <rama.nichanamatlu@...cle.com>
Subject: Re: [PATCH V4] mlx4_core: allocate ICM memory in page size chunks



On 6/4/2018 5:40 AM, Vlastimil Babka wrote:
> On 06/04/2018 08:27 AM, Michal Hocko wrote:
>> On Fri 01-06-18 15:05:26, Qing Huang wrote:
>>>
>>> On 6/1/2018 12:31 AM, Michal Hocko wrote:
>>>> On Thu 31-05-18 19:04:46, Qing Huang wrote:
>>>>> On 5/31/2018 2:10 AM, Michal Hocko wrote:
>>>>>> On Thu 31-05-18 10:55:32, Michal Hocko wrote:
>>>>>>> On Thu 31-05-18 04:35:31, Eric Dumazet wrote:
>>>>>> [...]
>>>>>>>> I merely copied/pasted from alloc_skb_with_frags() :/
>>>>>>> I will have a look at it. Thanks!
>>>>>> OK, so this is an example of an incremental development ;).
>>>>>>
>>>>>> __GFP_NORETRY was added by ed98df3361f0 ("net: use __GFP_NORETRY for
>>>>>> high order allocations") to prevent from OOM killer. Yet this was
>>>>>> not enough because fb05e7a89f50 ("net: don't wait for order-3 page
>>>>>> allocation") didn't want an excessive reclaim for non-costly orders
>>>>>> so it made it completely NOWAIT while it preserved __GFP_NORETRY in
>>>>>> place which is now redundant. Should I send a patch?
>>>>>>
>>>>> Just curious, how about GFP_ATOMIC flag? Would it work in a similar fashion?
>>>>> We experimented
>>>>> with it a bit in the past but it seemed to cause other issue in our tests.
>>>>> :-)
>>>> GFP_ATOMIC is a non-sleeping (aka no reclaim) context with an access to
>>>> memory reserves. So the risk is that you deplete those reserves and
>>>> cause issues to other subsystems which need them as well.
>>>>
>>>>> By the way, we didn't encounter any OOM killer events. It seemed that the
>>>>> mlx4_alloc_icm() triggered slowpath.
>>>>> We still had about 2GB free memory while it was highly fragmented.
>>>> The compaction was able to make a reasonable forward progress for you.
>>>> But considering mlx4_alloc_icm is called with GFP_KERNEL resp. GFP_HIGHUSER
>>>> then the OOM killer is clearly possible as long as the order is lower
>>>> than 4.
>>> The allocation was 256KB so the order was much higher than 4. The compaction
>>> seemed to be the root
>>> cause for our problem. It took too long to finish its work while putting
>>> mlx4_alloc_icm to sleep in a heavily
>>> fragmented memory situation . Will NORETRY flag avoid the compaction ops and
>>> fail the 256KB allocation
>>> immediately so mlx4_alloc_icm can enter adjustable lower order allocation
>>> code path quickly?
>> Costly orders should only perform a light compaction attempt unless
>> __GFP_RETRY_MAY_FAIL is used IIRC. CCing Vlastimil. So __GFP_NORETRY
>> shouldn't make any difference.
> It's a bit more complicated. Costly allocations will try the light
> compaction attempt first, even before reclaim. This is followed by
> reclaim and a more costly compaction attempt. With __GFP_NORETRY, the
> second compaction attempt is also only the light one, so the flag does
> make a difference here.

Thanks for the clarification!

Looks like our production kernel is kinda old, neither 
__GFP_DIRECT_RECLAIM nor __GFP_NORETRY
has been used in __alloc_pages_slowpath() in our kernel.