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

From: Vlastimil Babka
Date: Mon Jun 04 2018 - 08:40:45 EST

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.