Re: [PATCH 00/18] multiple preferred nodes

[Michal Hocko <mhocko@kernel.org>]

On Wed 24-06-20 13:23:44, Ben Widawsky wrote:
> On 20-06-24 22:07:50, Michal Hocko wrote:
> > On Wed 24-06-20 13:01:40, Ben Widawsky wrote:
> > > On 20-06-24 21:51:58, Michal Hocko wrote:
> > > > On Wed 24-06-20 12:37:33, Ben Widawsky wrote:
> > > > > On 20-06-24 20:39:17, Michal Hocko wrote:
> > > > > > On Wed 24-06-20 09:16:43, Ben Widawsky wrote:
> > [...]
> > > > > > > > Or do I miss something that really requires more involved approach like
> > > > > > > > building custom zonelists and other larger changes to the allocator?
> > > > > > > 
> > > > > > > I think I'm missing how this allows selecting from multiple preferred nodes. In
> > > > > > > this case when you try to get the page from the freelist, you'll get the
> > > > > > > zonelist of the preferred node, and when you actually scan through on page
> > > > > > > allocation, you have no way to filter out the non-preferred nodes. I think the
> > > > > > > plumbing of multiple nodes has to go all the way through
> > > > > > > __alloc_pages_nodemask(). But it's possible I've missed the point.
> > > > > > 
> > > > > > policy_nodemask() will provide the nodemask which will be used as a
> > > > > > filter on the policy_node.
> > > > > 
> > > > > Ah, gotcha. Enabling independent masks seemed useful. Some bad decisions got me
> > > > > to that point. UAPI cannot get independent masks, and callers of these functions
> > > > > don't yet use them.
> > > > > 
> > > > > So let me ask before I actually type it up and find it's much much simpler, is
> > > > > there not some perceived benefit to having both masks being independent?
> > > > 
> > > > I am not sure I follow. Which two masks do you have in mind? zonelist
> > > > and user provided nodemask?
> > > 
> > > Internally, a nodemask_t for preferred node, and a nodemask_t for bound nodes.
> > 
> > Each mask is a local to its policy object.
> 
> I mean for __alloc_pages_nodemask as an internal API. That is irrespective of
> policy. Policy decisions are all made beforehand. The question from a few mails
> ago was whether there is any use in keeping that change to
> __alloc_pages_nodemask accepting two nodemasks.

It is probably too late for me because I am still not following you
mean. Maybe it would be better to provide a pseudo code what you have in
mind. Anyway all that I am saying is that for the functionality that you
propose and _if_ the fallback strategy is fixed then all you should need
is to use the preferred nodemask for the __alloc_pages_nodemask and a
fallback allocation to the full (NULL nodemask). So you first try what
the userspace prefers - __GFP_RETRY_MAYFAIL will give you try hard but
do not OOM if the memory is depleted semantic and the fallback
allocation goes all the way to OOM on the complete memory depletion.
So I do not see much point in a custom zonelist for the policy. Maybe as
a micro-optimization to save some branches here and there.

If you envision usecases which might want to control the fallback
allocation strategy then this would get more complex because you
would need a sorted list of zones to try but this would really require
some solid usecase and it should build on top of a trivial
implementation which really is BIND with the fallback.

-- 
Michal Hocko
SUSE Labs