Re: [PATCH net-next v16 04/13] netdev: netdevice devmem allocator

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On Wed, Jul 10, 2024 at 12:55 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 10 Jul 2024 12:29:58 -0700 Mina Almasry wrote:
> > On Wed, Jul 10, 2024 at 9:37 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > > On Wed, 10 Jul 2024 00:17:37 +0000 Mina Almasry wrote:
> > > > +     net_devmem_dmabuf_binding_get(binding);
> > >
> > > Why does every iov need to hold a ref? pp holds a ref and does its own
> > > accounting, so it won't disappear unless all the pages are returned.
> >
> > I guess it doesn't really need to, but this is the design/approach I
> > went with, and I actually prefer it a bit. The design is borrowed from
> > how struct dev_pagemap does this, IIRC. Every page allocated from the
> > pgmap holds a reference to the pgmap to ensure the pgmap doesn't go
> > away while some page that originated from it is out in the wild, and
> > similarly I did so in the binding here.
>
> Oh, you napi_pp_put_page() on the other end! I can see how that could
> be fine.
>
> > We could assume that the page_pool is accounting iovs for us, but that
> > is not always true, right? page_pool_return_page() disconnects a
> > netmem from the page_pool and AFAIU the page_pool can go away while
> > there is such a netmem still in use in the net stack. Currently this
> > can't happen with iovs because I currently don't support non-pp
> > refcounting for iovs (so they're always recyclable), but you have a
> > comment on the other patch asking why that works; depending on how we
> > converge on that conversation, the details of how the pp refcounting
> > could change.
>
> Even then - we could take the ref as the page "leaks" out of the pool,
> rather than doing it on the fast path, right? Or just BUG_ON() 'cause
> that reference ain't coming back ;)
>

OK, I'll see how the conversation on the other thread converges
vis-a-vis net_iov refcounting happens, and then look at if I can avoid
the binding_get/put per page in that framework.

> > It's nice to know that the binding refcounting will work regardless of
> > the details of how the pp refcounting works. IMHO having the binding
> > rely on the pp refcounting to ensure all the iovs are freed introduces
> > some fragility.
> >
> > Additionally IMO the net_devmem_dmabuf_binding_get/put aren't so
> > expensive to want to optimize out, right? The allocation is a slow
> > path anyway and the fast path recycles netmem.
>
> Yes, I should have read patch 10. I think it's avoidable :) but with
> recycling it can indeed perform just fine (do you happen to have
> recycling rate stats from prod runs?)

I don't to be honest. For a couple of reasons, one is that gcloud VMs
where we mainly use this, these stats are private to the VM and is not
something I can query widly. I only get access to the data when shared
with bug reports on specific issues.

In our internal test runs, I do not monitor the recycling rate to be
honest, as that is fine as long as the recycling is fast enough to
find available memory for incoming data. What I do look at very
closely is the allocation failure rate. That is when GVE tries to
alloc a new devmem but it's out of devmem (which would likely be due
to recycling not happening fast enough). The stat is `page_alloc_fail`
in ethtool -S for us and it's one of the first things I check when
things go wrong. It hasn't been the root cause for any of our issues
in reality.

--
Thanks,
Mina




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