linux-kernel - Re: [RFC PATCH rdma-next 08/10] RDMA/rxe: Implement flush execution in responder side

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Message-ID: <20220106003525.GT6467@ziepe.ca>
Date:   Wed, 5 Jan 2022 20:35:25 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Tom Talpey <tom@...pey.com>
Cc:     "lizhijian@...itsu.com" <lizhijian@...itsu.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "zyjzyj2000@...il.com" <zyjzyj2000@...il.com>,
        "aharonl@...dia.com" <aharonl@...dia.com>,
        "leon@...nel.org" <leon@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mbloch@...dia.com" <mbloch@...dia.com>,
        "liweihang@...wei.com" <liweihang@...wei.com>,
        "liangwenpeng@...wei.com" <liangwenpeng@...wei.com>,
        "yangx.jy@...itsu.com" <yangx.jy@...itsu.com>,
        "rpearsonhpe@...il.com" <rpearsonhpe@...il.com>,
        "y-goto@...itsu.com" <y-goto@...itsu.com>
Subject: Re: [RFC PATCH rdma-next 08/10] RDMA/rxe: Implement flush execution
 in responder side

On Thu, Dec 30, 2021 at 09:32:06PM -0500, Tom Talpey wrote:

> The global visibility is oriented toward supporting distributed
> shared memory workloads, or publish/subscribe on high scale systems.
> It's mainly about ensuring that all devices and CPUs see the data,
> something ordinary RDMA Write does not guarantee. This often means
> flushing write pipelines, possibly followed by invalidating caches.

Isn't that what that new ATOMIC_WRITE does? Why do I need to flush if
ATOMIC WRITE was specified as a release? All I need to do is acquire
on the ATOMIC_WRITE site and I'm good?

So what does FLUSH do here, and how does a CPU 'acquire' against this
kind of flush? The flush doesn't imply any ordering right, so how is
it useful on its own?

The write to persistance aspect I understand, but this notion of
global viability seems peculiar.

> Well, higher level wrappers may signal errors, for example if they're
> not available or unreliable, and you will need to handle them. Also,
> they may block. Is that ok in this context?

I'm not sure we have any other tools here beyond a release barrier
like wmb() or the special pmem cache flush. Neither are blocking or
can fail.

In pmem systems storage failure is handled via the memory failure
stuff asynchronously.

Jason