netdev - Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE

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Message-ID: <782776.1678987682@warthog.procyon.org.uk>
Date:   Thu, 16 Mar 2023 17:28:02 +0000
From:   David Howells <dhowells@...hat.com>
To:     Chuck Lever III <chuck.lever@...cle.com>
Cc:     dhowells@...hat.com, Trond Myklebust <trondmy@...merspace.com>,
        Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Jeffrey Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Anna Schumaker <anna@...nel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>
Subject: Re: [RFC PATCH 27/28] sunrpc: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

Chuck Lever III <chuck.lever@...cle.com> wrote:

> That means I haven't seen the cover letter and do not have any
> context for this proposed change.

https://lore.kernel.org/linux-fsdevel/20230316152618.711970-1-dhowells@redhat.com/

> We've tried combining the sendpages calls in here before. It
> results in a significant and measurable performance regression.
> See:
> 
> da1661b93bf4 ("SUNRPC: Teach server to use xprt_sock_sendmsg for socket sends")

The commit replaced the use of sendpage with sendmsg, but that took away the
zerocopy aspect of sendpage.  The idea behind MSG_SPLICE_PAGES is that it
allows you to do keep that.  I'll have to try reapplying this commit and
adding the MSG_SPLICE_PAGES flag.

> Therefore, this kind of change needs to be accompanied by both
> benchmark results and some field testing to convince me it won't
> cause harm.

Yep.

> And, we have to make certain that this doesn't break operation
> with kTLS sockets... do they support MSG_SPLICE_PAGES ?

I haven't yet tackled AF_TLS, AF_KCM or AF_SMC as they seem significantly more
complex than TCP and UDP.  I thought I'd get some feedback on what I have
before I tried my hand at those.

David