linux-kernel - Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED

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Date:   Thu, 20 Aug 2020 10:42:05 -0700
From:   Peter Oskolkov <posk@...k.io>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Peter Oskolkov <posk@...gle.com>, paulmck <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

On Wed, Aug 12, 2020 at 12:44 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
[...]
>
> > One way of doing what you suggest is to allow some commands to be bitwise-ORed.
> >
> > So, for example, the user could call
> >
> > membarrier(CMD_PRIVATE_EXPEDITED_SYNC_CORE | CMD_PRIVATE_EXPEDITED_RSEQ, cpu_id)
> >
> > Is this what you have in mind?
>
> Not really. This would not take care of the fact that we would end up multiplying
> the number of commands as we allow combinations. E.g. if we ever want to have RSEQ
> work in private and global, and in non-expedited and expedited, we end up needing:
>
> - CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
> - CMD_PRIVATE_EXPEDITED_RSEQ
> - CMD_PRIVATE_RSEQ
> - CMD_REGISTER_GLOBAL_EXPEDITED_RSEQ
> - CMD_GLOBAL_EXPEDITED_RSEQ
> - CMD_GLOBAL_RSEQ
>
> The only thing we would save by OR'ing it with the SYNC_CORE command is the additional
> list:
>
> - CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ_SYNC_CORE
> - CMD_PRIVATE_EXPEDITED_RSEQ_SYNC_CORE
> - CMD_PRIVATE_RSEQ_SYNC_CORE
> - CMD_REGISTER_GLOBAL_EXPEDITED_RSEQ_SYNC_CORE
> - CMD_GLOBAL_EXPEDITED_RSEQ_SYNC_CORE
> - CMD_GLOBAL_RSEQ_SYNC_CORE
>
> But unless we receive feedback that doing a membarrier with RSEQ+sync_core all in
> one go is a significant use-case, I am tempted to leave out that scenario for now.
> If we go for new commands, this means we could add (for private-expedited-rseq):
>
> - MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ,
> - MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ,
>
> I do however have use-cases for using RSEQ across shared memory (between
> processes). Not currently for a rseq-fence, but for rseq acting as per-cpu
> atomic operations. If I ever end up needing rseq-fence across shared memory,
> that would result in the following new commands:
>
> - MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED_RSEQ,
> - MEMBARRIER_CMD_GLOBAL_EXPEDITED_RSEQ,
>
> The remaining open question is whether it would be OK to define a new
> membarrier flag=MEMBARRIER_FLAG_CPU, which would expect an additional
> @cpu parameter.

Hi  Mathieu,

I do not think there is any reason to wait for additional feedback, so I believe
we should finalize the API/ABI.

I see two issues to resolve:
1: how to combine commands:
  - you do not like adding new commands that are combinations of existing ones;
  - you do not like ORing.
=> I'm not sure what other options we have here?

2: should @flags be repurposed for cpu_id, or MEMBARRIER_FLAG_CPU
   added with a new syscall parameter.
=> I'm still not sure a new parameter can be cleanly added, but I can try
   it in the next patchset if you prefer it this way.

Please let me know your thoughts.

Thanks,
Peter