linux-kernel - Re: [GIT PULL] RCU changes for v2.6.38

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Message-ID: <20110107083240.GN2506@linux.vnet.ibm.com>
Date:	Fri, 7 Jan 2011 00:32:41 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU changes for v2.6.38

On Fri, Jan 07, 2011 at 12:54:57AM +0100, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Thu, Jan 6, 2011 at 1:08 AM, Ingo Molnar <mingo@...e.hu> wrote:
> > >
> > > Please pull the latest core-rcu-for-linus git tree from:
> > 
> > So I pulled, but I'm not convinced about the crazy busy-looping
> > SRCU_SYNCHRONIZE_DELAY thing.
> > 
> > Why does it do a silly udelay(), instead of just looping over the
> > srcu_readers_active_idx() for a few times? You're wasting CPU time
> > anyway, why ask the user how many usecs to waste?
> > 
> > IOW, why isn't that "wait for no active readers" a nice helper
> > function, and why doesn't it do
> > 
> >    for (i = 0; i < CONFIG_SRCU_SYNCHRONIZE_DELAY; i++) {
> >       if (!srcu_readers_active_idx(sp, idx))
> >          return;
> >       udelay(1);
> >    }
> >    while (srcu_readers_active_idx(sp, idx))
> >       schedule_timeout_interruptible(1);
> > 
> > instead? And is it really sane to ask the kernel configurator to come
> > up with a random value (ie that "CONFIG_SRCU_SYNCHRONIZE_DELAY" is
> > just stupid and wrong)?
> > 
> > Please fix this. And don't make people answer unanswerable questions.
> > If YOU and Paul don't know the answer, why the hell do you expect
> > somebody who does a "make config" to know the answer?
> > 
> > Either pick a number, or pick an algorithm that self-tunes.
> > 
> > Don't use the Kconfig system as a way to tell people that it's their
> > fault when you made a bad decision. Really.
> 
> Fully agreed - we'll fix this.

I am on it!

							Thanx, Paul
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