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From: "Andy Lutomirski" <luto@kernel.org>
To: "Peter Zijlstra (Intel)" <peterz@infradead.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
Date: Tue, 14 Sep 2021 11:40:01 -0700	[thread overview]
Message-ID: <f6fdecfe-963d-4669-ae05-1d7192467a19@www.fastmail.com> (raw)
In-Reply-To: <YUDlzxLjNsW+oYGC@hirez.programming.kicks-ass.net>



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.

> 
> > 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?

  reply	other threads:[~2021-09-14 18:40 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 18:49 [PATCH 0/4 v0.5] sched/umcg: RFC UMCG patchset Peter Oskolkov
2021-09-08 18:49 ` [PATCH 1/4 v0.5] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-09-08 18:49 ` [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-09-08 23:38   ` Jann Horn
2021-09-09  1:16     ` Jann Horn
2021-09-09 19:06     ` Peter Oskolkov
2021-09-09 21:20       ` Jann Horn
2021-09-09 22:09         ` Peter Oskolkov
2021-09-09 23:13           ` Jann Horn
2021-09-14 16:52         ` Andy Lutomirski
2021-09-14 18:11           ` Peter Zijlstra
2021-09-14 18:40             ` Andy Lutomirski [this message]
2021-09-15 15:42               ` Peter Zijlstra
2021-09-15 16:50                 ` Andy Lutomirski
2021-09-15 19:10                   ` Peter Zijlstra
2021-09-14  8:07       ` Peter Zijlstra
2021-09-14 16:29         ` Peter Oskolkov
2021-09-14 18:04           ` Peter Zijlstra
2021-09-14 18:15             ` Peter Zijlstra
2021-09-14 18:29             ` Peter Oskolkov
2021-09-14 18:48               ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-09-09  1:39   ` Jann Horn
2021-09-14 16:51     ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 4/4 v0.5] sched/umcg: add Documentation/userspace-api/umcg.rst Peter Oskolkov
2021-09-14 16:35   ` Tao Zhou
2021-09-14 16:57     ` Peter Oskolkov

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