Re: [RFC PATCH v4 7/7] mm/demotion: Demote pages according to allocation fallback order

Aneesh Kumar K V
2022-06-06T05:03:13Z

On 6/6/22 6:13 AM, Ying Huang wrote:
> On Fri, 2022-06-03 at 20:39 +0530, Aneesh Kumar K V wrote:
>> On 6/2/22 1:05 PM, Ying Huang wrote:
>>> On Fri, 2022-05-27 at 17:55 +0530, Aneesh Kumar K.V wrote:
>>>> From: Jagdish Gediya <jvgediya@linux.ibm.com>
>>>>
>>>> currently, a higher tier node can only be demoted to selected
>>>> nodes on the next lower tier as defined by the demotion path,
>>>> not any other node from any lower tier. This strict, hard-coded
>>>> demotion order does not work in all use cases (e.g. some use cases
>>>> may want to allow cross-socket demotion to another node in the same
>>>> demotion tier as a fallback when the preferred demotion node is out
>>>> of space). This demotion order is also inconsistent with the page
>>>> allocation fallback order when all the nodes in a higher tier are
>>>> out of space: The page allocation can fall back to any node from any
>>>> lower tier, whereas the demotion order doesn't allow that currently.
>>>>
>>>> This patch adds support to get all the allowed demotion targets mask
>>>> for node, also demote_page_list() function is modified to utilize this
>>>> allowed node mask by filling it in migration_target_control structure
>>>> before passing it to migrate_pages().
>>>
>>
>> ...
>>
>>>>    * Take pages on @demote_list and attempt to demote them to
>>>>    * another node. Pages which are not demoted are left on
>>>> @@ -1481,6 +1464,19 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
>>>>   {
>>>>    int target_nid = next_demotion_node(pgdat->node_id);
>>>>    unsigned int nr_succeeded;
>>>> + nodemask_t allowed_mask;
>>>> +
>>>> + struct migration_target_control mtc = {
>>>> + /*
>>>> + * Allocate from 'node', or fail quickly and quietly.
>>>> + * When this happens, 'page' will likely just be discarded
>>>> + * instead of migrated.
>>>> + */
>>>> + .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) | __GFP_NOWARN |
>>>> + __GFP_NOMEMALLOC | GFP_NOWAIT,
>>>> + .nid = target_nid,
>>>> + .nmask = &allowed_mask
>>>> + };
>>>
>>> IMHO, we should try to allocate from preferred node firstly (which will
>>> kick kswapd of the preferred node if necessary). If failed, we will
>>> fallback to all allowed node.
>>>
>>> As we discussed as follows,
>>>
>>> https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@intel.com/
>>>
>>> That is, something like below,
>>>
>>> static struct page *alloc_demote_page(struct page *page, unsigned long node)
>>> {
>>> struct page *page;
>>> nodemask_t allowed_mask;
>>> struct migration_target_control mtc = {
>>> /*
>>> * Allocate from 'node', or fail quickly and quietly.
>>> * When this happens, 'page' will likely just be discarded
>>> * instead of migrated.
>>> */
>>> .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>>> __GFP_THISNODE | __GFP_NOWARN |
>>> __GFP_NOMEMALLOC | GFP_NOWAIT,
>>> .nid = node
>>> };
>>>
>>> page = alloc_migration_target(page, (unsigned long)&mtc);
>>> if (page)
>>> return page;
>>>
>>> mtc.gfp_mask &= ~__GFP_THISNODE;
>>> mtc.nmask = &allowed_mask;
>>>
>>> return alloc_migration_target(page, (unsigned long)&mtc);
>>> }
>>
>> I skipped doing this in v5 because I was not sure this is really what we
>> want.
>
> I think so. And this is the original behavior. We should keep the
> original behavior as much as possible, then make changes if necessary.
>

That is the reason I split the new page allocation as a separate patch.
Previous discussion on this topic didn't conclude on whether we really
need to do the above or not
https://lore.kernel.org/lkml/CAAPL-u9endrWf_aOnPENDPdvT-2-YhCAeJ7ONGckGnXErTLOfQ@mail.gmail.com/

Based on the above I looked at avoiding GFP_THISNODE allocation. If you
have experiment results that suggest otherwise can you share? I could
summarize that in the commit message for better description of why
GFP_THISNODE enforcing is needed.

>> I guess we can do this as part of the change that is going to
>> introduce the usage of memory policy for the allocation?
>
> Like the memory allocation policy, the default policy should be local
> preferred. We shouldn't force users to use explicit memory policy for
> that.
>
> And the added code isn't complex.
>

-aneesh