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

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Message-ID: <20120131222447.GH2391@linux.vnet.ibm.com>
Date:	Tue, 31 Jan 2012 14:24:47 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.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 Tue, Jan 31, 2012 at 02:50:07PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-01-31 at 15:47 +0200, Avi Kivity wrote:
> 
> > They really need to return quickly to userspace, and they really need to
> > perform some operation between rcu_assign_pointer() and returning, so no.
> 
> Bugger :/
> 
> > >
> > > Compile tested only!! :-)
> > >
> > 
> > How much did synchronize_srcu_expedited() regress?  Presumably your
> > compiler didn't tell you that.
> 
> Nope, quite a lot I figure.
> 
> > 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.
> 
> OK, I'll try and come up with means of making it go fast again ;-)

I cannot resist suggesting a kthread to do the call_srcu(), which
would allow synchronize_srcu_expedited() to proceed with its current
brute-force speed.

							Thanx, Paul

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