linux-kernel - Re: [RFC][PATCH 12/13] stop

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Message-ID: <20150624152705.GE3717@linux.vnet.ibm.com>
Date:	Wed, 24 Jun 2015 08:27:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

On Wed, Jun 24, 2015 at 05:01:51PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 24, 2015 at 07:50:42AM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 24, 2015 at 09:35:03AM +0200, Peter Zijlstra wrote:
> 
> > > I still don't see a problem here though; the stop_one_cpu() invocation
> > > for the CPU that's suffering its preemption latency will take longer,
> > > but so what?
> > > 
> > > How does polling and dropping back to sync_rcu() generate better
> > > behaviour than simply waiting for the completion?
> > 
> > Because if there is too much delay, synchronize_rcu() is no slower
> > than is synchronize_rcu_expedited(), plus synchronize_rcu() is much
> > more efficient.
> 
> Still confused.. How is polling and then blocking more efficient than
> just blocking in the first place? I'm seeing the polling as a waste of
> cpu time.

As I said, the current code is quite old and will get a facelift.

> The thing is, if we're stalled on a stop_one_cpu() call, the sync_rcu()
> is equally stalled. The sync_rcu() cannot wait more efficient than we're
> already waiting either.

Ah, but synchronize_rcu() doesn't force waiting on more than one extra
grace period.  With strictly queued mutex, you can end up waiting on
several.

							Thanx, Paul

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