Re: [PATCH] mm, thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings

["Zi Yan" <zi.yan@cs.rutgers.edu>]

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On 30 Aug 2018, at 9:45, Michal Hocko wrote:

> On Thu 30-08-18 09:22:21, Zi Yan wrote:
>> On 30 Aug 2018, at 3:00, Michal Hocko wrote:
>>
>>> On Wed 29-08-18 18:54:23, Zi Yan wrote:
>>> [...]
>>>> I tested it against Linus’s tree with “memhog -r3 130g” in a two-socket machine with 128GB memory on
>>>> each node and got the results below. I expect this test should fill one node, then fall back to the other.
>>>>
>>>> 1. madvise(MADV_HUGEPAGE) + defrag = {always, madvise, defer+madvise}:
>>>> no swap, THPs are allocated in the fallback node.
>>>> 2. madvise(MADV_HUGEPAGE) + defrag = defer: pages got swapped to the
>>>> disk instead of being allocated in the fallback node.
>>>> 3. no madvise, THP is on by default + defrag = {always, defer,
>>>> defer+madvise}: pages got swapped to the disk instead of being
>>>> allocated in the fallback node.
>>>> 4. no madvise, THP is on by default + defrag = madvise: no swap, base
>>>> pages are allocated in the fallback node.
>>>>
>>>> The result 2 and 3 seems unexpected, since pages should be allocated in the fallback node.
>>>>
>>>> The reason, as Andrea mentioned in his email, is that the combination
>>>> of __THIS_NODE and __GFP_DIRECT_RECLAIM (plus __GFP_KSWAPD_RECLAIM
>>>> from this experiment).
>>>
>>> But we do not set __GFP_THISNODE along with __GFP_DIRECT_RECLAIM AFAICS.
>>> We do for __GFP_KSWAPD_RECLAIM though and I guess that it is expected to
>>> see kswapd do the reclaim to balance the node. If the node is full of
>>> anonymous pages then there is no other way than swap out.
>>
>> GFP_TRANSHUGE implies __GFP_DIRECT_RECLAIM. When no madvise is given, THP is on
>> + defrag=always, gfp_mask has __GFP_THISNODE and __GFP_DIRECT_RECLAIM, so swapping
>> can be triggered.
>
> Yes, but the setup tells that you are willing to pay price to get a THP.
> defered=always uses that special __GFP_NORETRY (unless it is madvised
> mapping) that should back off if the compaction failed recently. How
> much that reduces the reclaim is not really clear to me right now to be
> honest.
>
>> The key issue here is that “memhog -r3 130g” uses the default memory policy (MPOL_DEFAULT),
>> which should allow page allocation fallback to other nodes, but as shown in
>> result 3, swapping is triggered instead of page allocation fallback.
>
> Well, I guess this really depends. Fallback to a different node might be
> seen as a bad thing and worse than the reclaim on the local node.
>
>>>> __THIS_NODE uses ZONELIST_NOFALLBACK, which
>>>> removes the fallback possibility and __GFP_*_RECLAIM triggers page
>>>> reclaim in the first page allocation node when fallback nodes are
>>>> removed by ZONELIST_NOFALLBACK.
>>>
>>> Yes but the point is that the allocations which use __GFP_THISNODE are
>>> optimistic so they shouldn't fallback to remote NUMA nodes.
>>
>> This can be achieved by using MPOL_BIND memory policy which restricts
>> nodemask in struct alloc_context for user space memory allocations.
>
> Yes, but that requires and explicit NUMA handling. And we are trying to
> handle those cases which do not really give a damn and just want to use
> THP if it is available or try harder when they ask by using madvise.
>
>>>> IMHO, __THIS_NODE should not be used for user memory allocation at
>>>> all, since it fights against most of memory policies.  But kernel
>>>> memory allocation would need it as a kernel MPOL_BIND memory policy.
>>>
>>> __GFP_THISNODE is indeed an ugliness. I would really love to get rid of
>>> it here. But the problem is that optimistic THP allocations should
>>> prefer a local node because a remote node might easily offset the
>>> advantage of the THP. I do not have a great idea how to achieve that
>>> without __GFP_THISNODE though.
>>
>> MPOL_PREFERRED memory policy can be used to achieve this optimistic
>> THP allocation for user space. Even with the default memory policy,
>> local memory node will be used first until it is full. It seems to
>> me that __GFP_THISNODE is not necessary if a proper memory policy is
>> used.
>>
>> Let me know if I miss anything. Thanks.
>
> You are missing that we are trying to define a sensible model for those
> who do not really care about mempolicies. THP shouldn't cause more harm
> than good for those.
>
> I wish we could come up with a remotely sane and comprehensible model.
> That means that you know how hard the allocator tries to get a THP for
> you depending on the defrag configuration, your memory policy and your
> madvise setting. The easiest one I can think of is to
> - always follow mempolicy when specified because you asked for it
>   explicitly
> - stay node local and low latency for the light THP defrag mode (defrag,
>   madvise without hint and none) because THP is a nice to have
> - if the defrag mode is always then you are willing to pay the latency
>   price but off-node might be still a no-no.
> - allow fallback for madvised mappings because you really want THP. If
>   you care about specific numa placement then combine with the
>   mempolicy.
>
> As you can see I do not really mention anything about the direct reclaim
> because that is just an implementation detail of the page allocator and
> compaction interaction.
>
> Maybe you can formulate a saner matrix with all the available modes that
> we have.
>
> Anyway, I guess we can agree that (almost) unconditional __GFP_THISNODE
> is clearly wrong and we should address that first. Either Andrea's
> option 2) patch or mine which does the similar thing except at the
> proper layer (I believe). We can continue discussing other odd cases on
> top I guess. Unless somebody has much brighter idea, of course.

Thanks for your explanation. It makes sense to me. I am fine with your patch.
You can add my Tested-by: Zi Yan <zi.yan@cs.rutgers.edu>, since
my test result 1 shows that the problem mentioned in your changelog is solved.

—
Best Regards,
Yan Zi

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