netdev - Re: [PATCH] xsk: add cq event

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Message-ID: <90c58756-934d-adf6-64e8-680cfc019cd4@intel.com>
Date:   Mon, 16 Nov 2020 15:31:20 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xsk: add cq event

On 2020-11-16 09:10, Xuan Zhuo wrote:
> When we write all cq items to tx, we have to wait for a new event based
> on poll to indicate that it is writable. But the current writability is
> triggered based on whether tx is full or not, and In fact, when tx is
> dissatisfied, the user of cq's item may not necessarily get it, because it
> may still be occupied by the network card. In this case, we need to know
> when cq is available, so this patch adds a socket option, When the user
> configures this option using setsockopt, when cq is available, a
> readable event is generated for all xsk bound to this umem.
> 
> I can't find a better description of this event,
> I think it can also be 'readable', although it is indeed different from
> the 'readable' of the new data. But the overhead of xsk checking whether
> cq or rx is readable is small.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>

Thanks for the patch!

I'm not a fan of having two different "readable" event (both Rx and cq).
Could you explain a bit what the use case is, so I get a better
understanding.

The Tx queues has a back-pressure mechanism, determined of the number of
elements in cq. Is it related to that?

Please explain a bit more what you're trying to solve, and maybe we can
figure out a better way forward!


Thanks!
Björn