netdev - Re: [RFC PATCH] ptr_ring: linked list fallback

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Date:   Fri, 16 Feb 2018 16:32:05 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mst@...hat.com
Cc:     linux-kernel@...r.kernel.org, john.fastabend@...il.com,
        netdev@...r.kernel.org, jasowang@...hat.com
Subject: Re: [RFC PATCH] ptr_ring: linked list fallback

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Fri, 16 Feb 2018 09:40:54 +0200

> So pointer rings work fine, but they have a problem:
> make them too small and not enough entries fit.
> Make them too large and you start flushing your cache
> and running out of memory.
> 
> This is a new idea of mine: a ring backed by a
> linked list. Once you run out of rin entries,
> instead of a drop you fall back on a list with
> a common lock.
> 
> Should work well for the case where the ring is typically sized
> correctly, but will help address the fact that some user try to set e.g.
> tx queue length to 1000000.
> 
> My hope this will move us closer to direction where e.g. fw codel can
> use ptr rings without locking at all.
> The API is still very rough, and I really need to take a hard look
> at lock nesting.
> 
> Completely untested, sending for early feedback/flames.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

So the idea is that if a user sets a really huge TX queue length, we allocate
a ptr_ring which is smaller, and use the backup linked list when necessary
to provide the requested TX queue length legitimately.

Right?