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Date:   Wed, 15 Sep 2021 17:42:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Jann Horn <jannh@google.com>, Peter Oskolkov <posk@google.com>,
        Peter Oskolkov <posk@posk.io>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Andrei Vagin <avagin@google.com>,
        Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
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On Tue, Sep 14, 2021 at 11:40:01AM -0700, Andy Lutomirski wrote:
> 
> 
> On Tue, Sep 14, 2021, at 11:11 AM, Peter Zijlstra wrote:
> > On Tue, Sep 14, 2021 at 09:52:08AM -0700, Andy Lutomirski wrote:
> > > With a custom mapping, you don’t need to pin pages at all, I think.
> > > As long as you can reconstruct the contents of the shared page and
> > > you’re willing to do some slightly careful synchronization, you can
> > > detect that the page is missing when you try to update it and skip the
> > > update. The vm_ops->fault handler can repopulate the page the next
> > > time it’s accessed.
> > 
> > The point is that the moment we know we need to do this user-poke, is
> > schedule(), which could be called while holding mmap_sem (it being a
> > preemptable lock). Which means we cannot go and do faults.
> 
> That’s fine. The page would be in one or two states: present and
> writable by kernel or completely gone. If its present, the scheduler
> writes it. If it’s gone, the scheduler skips the write and the next
> fault fills it in.

That's non-deterministic, and as such not suitable.

> > > All that being said, I feel like I’m missing something. The point of
> > > this is to send what the old M:N folks called “scheduler activations”,
> > > right?  Wouldn’t it be more efficient to explicitly wake something
> > > blockable/pollable and write the message into a more efficient data
> > > structure?  Polling one page per task from userspace seems like it
> > > will have inherently high latency due to the polling interval and will
> > > also have very poor locality.  Or am I missing something?
> > 
> > The idea was to link the user structures together in a (single) linked
> > list. The server structure gets a list of all the blocked tasks. This
> > avoids having to a full N iteration (like Java, they're talking stupid
> > number of N).
> > 
> > Polling should not happen, once we run out of runnable tasks, the server
> > task gets ran again and it can instantly pick up all the blocked
> > notifications.
> > 
> 
> How does the server task know when to read the linked list?  And
> what’s wrong with a ring buffer or a syscall?

Same problem, ring-buffer has the case where it's full and events get
dropped, at which point you've completely lost state. If it is at all
possible to recover from that, doing so is non-deterministic.

I really want this stuff to work for realtime workloads too.