netdev - Re: [PATCH][v2] page_pool: handle page recycle for NUMA_NO

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Date:   Mon, 16 Dec 2019 15:21:28 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        Li Rongqing <lirongqing@...du.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        peterz@...radead.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        bhelgaas@...gle.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][v2] page_pool: handle page recycle for NUMA_NO_NODE
 condition

On Mon, Dec 16, 2019 at 02:08:45PM +0100, Michal Hocko wrote:
> On Mon 16-12-19 14:34:26, Ilias Apalodimas wrote:
> > Hi Michal, 
> > On Mon, Dec 16, 2019 at 01:15:57PM +0100, Michal Hocko wrote:
> > > On Thu 12-12-19 09:34:14, Yunsheng Lin wrote:
> > > > +CC Michal, Peter, Greg and Bjorn
> > > > Because there has been disscusion about where and how the NUMA_NO_NODE
> > > > should be handled before.
> > > 
> > > I do not have a full context. What is the question here?
> > 
> > When we allocate pages for the page_pool API, during the init, the driver writer
> > decides which NUMA node to use. The API can,  in some cases recycle the memory,
> > instead of freeing it and re-allocating it. If the NUMA node has changed (irq
> > affinity for example), we forbid recycling and free the memory, since recycling
> > and using memory on far NUMA nodes is more expensive (more expensive than
> > recycling, at least on the architectures we tried anyway).
> > Since this would be expensive to do it per packet, the burden falls on the 
> > driver writer for that. Drivers *have* to call page_pool_update_nid() or 
> > page_pool_nid_changed() if they want to check for that which runs once
> > per NAPI cycle.
> 
> Thanks for the clarification.
> 
> > The current code in the API though does not account for NUMA_NO_NODE. That's
> > what this is trying to fix.
> > If the page_pool params are initialized with that, we *never* recycle
> > the memory. This is happening because the API is allocating memory with 
> > 'nid = numa_mem_id()' if NUMA_NO_NODE is configured so the current if statement
> > 'page_to_nid(page) == pool->p.nid' will never trigger.
> 
> OK. There is no explicit mention of the expected behavior for
> NUMA_NO_NODE. The semantic is usually that there is no NUMA placement
> requirement and the MM code simply starts the allocate from a local node
> in that case. But the memory might come from any node so there is no
> "local node" guarantee.
> 
> So the main question is what is the expected semantic? Do people expect
> that NUMA_NO_NODE implies locality? Why don't you simply always reuse
> when there was no explicit numa requirement?
> 

Well they shouldn't. Hence my next proposal. I think we are pretty much saying
the same thing here. 
If the driver defines NUMA_NO_NODE, just blindly recycle memory.

> > The initial proposal was to check:
> > pool->p.nid == NUMA_NO_NODE && page_to_nid(page) == numa_mem_id()));
> 
> > After that the thread span out of control :)
> > My question is do we *really* have to check for 
> > page_to_nid(page) == numa_mem_id()? if the architecture is not NUMA aware
> > wouldn't pool->p.nid == NUMA_NO_NODE be enough?
> 
> If the architecture is !NUMA then numa_mem_id and page_to_nid should
> always equal and be both zero.
> 

Ditto

> -- 
> Michal Hocko
> SUSE Labs


Thanks
/Ilias