linux-kernel - Re: [RFC][PATCH] srcu: Implement call

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Date:	Tue, 31 Jan 2012 15:47:18 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [RFC][PATCH] srcu: Implement call_srcu()

On 01/31/2012 03:32 PM, Peter Zijlstra wrote:
> Subject: srcu: Implement call_srcu()
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Mon Jan 30 23:20:49 CET 2012
>
> Implement call_srcu() by using a state machine driven by
> call_rcu_sched() and timer callbacks.
>
> The state machine is a direct derivation of the existing
> synchronize_srcu() code and replaces synchronize_sched() calls with a
> call_rcu_sched() callback and the schedule_timeout() calls with simple
> timer callbacks.
>
> It then re-implements synchronize_srcu() using a completion where we
> send the complete through call_srcu().
>
> It completely wrecks synchronize_srcu_expedited() which is only used
> by KVM. 3 of the 5 

2 of the 5

> use cases look like they really want to use
> call_srcu() instead, the remaining 2 I don't know but hope they can,
> which would let us remove it.

They really need to return quickly to userspace, and they really need to
perform some operation between rcu_assign_pointer() and returning, so no.

>
> Compile tested only!! :-)
>

How much did synchronize_srcu_expedited() regress?  Presumably your
compiler didn't tell you that.

Can we get it back to speed by scheduling a work function on all cpus? 
wouldn't that force a quiescent state and allow call_srcu() to fire?

In kvm's use case synchronize_srcu_expedited() is usually called when no
thread is in a critical section, so we don't have to wait for anything
except the srcu machinery.

-- 
error compiling committee.c: too many arguments to function

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