linux-kernel - Re: Tasks RCU vs Preempt RCU

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Message-ID: <20180522083832.45353d5f@gandalf.local.home>
Date:   Tue, 22 May 2018 08:38:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        byungchul.park@....com, mathieu.desnoyers@...icios.com,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: Tasks RCU vs Preempt RCU

On Mon, 21 May 2018 21:54:14 -0700
Joel Fernandes <joel@...lfernandes.org> wrote:

> Yes, lets brain storm this if you like. One way I was thinking if we can
> manually check every CPU and see what state its in (usermode, kernel, idle
> etc) using an IPI mechanism. Once all CPUs have been seen to be in usermode,
> or idle atleast once - then we are done. You have probably already thought

Nope, it has nothing to do with CPUs, it really has to do with tasks.

	CPU0
	----
 task 1: (pinned to CPU 0)
	 call func_tracer_trampoline
	 [on trampoline]
	 [timer tick, schedule ]

 task 2: (higher priority, also pinned to CPU 0)
	 goes to user space
	 [ Runs for along time ]

We cannot free the trampoline until task 2 releases the CPU and lets
task 1 run again to get off the CPU.

> about this so feel free to say why its not a good idea, but to me there are 3
> places that a tasks quiescent state is recorded: during the timer tick,
> during task sleep and during rcu_note_voluntary_context_switch in
> cond_resched_rcu_qs. Of these, I feel only the cond_resched_rcu_qs case isn't
> trackable with IPI mechanism which may make the detection a bit slower, but
> tasks-RCU in mainline is slow right now anyway (~ 1 second delay if any task
> was held).

The way I was originally going to handle this was with a per task
counter, where it can be incremented at certain points via tracepoints.

Thus my synchronize tasks, would have connected to a bunch of
tracepoints at known quiescent states that would increment the counter,
and then check each task until they all pass a certain point, or are in
a quiescent state (userspace or idle). But this would be doing much of
what RCU does today, and that is why we decided to hook with the RCU
infrastructure.

I have to ask, what's your motivation for getting rid of RCU tasks?

-- Steve