linux-kernel - Re: [PATCH tip/core/rcu 0/5] Fix for cond

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Message-ID: <20140620212423.GA22886@cloud>
Date:	Fri, 20 Jun 2014 14:24:23 -0700
From:	josh@...htriplett.org
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 0/5] Fix for cond_resched performance
 regression

On Fri, Jun 20, 2014 at 12:12:36PM -0700, Paul E. McKenney wrote:
> o	Make cond_resched() a no-op for PREEMPT=y.  This might well turn
> 	out to be a good thing, but it doesn't help give RCU the quiescent
> 	states that it needs.

What about doing this, together with letting the fqs logic poke
un-quiesced kernel code as needed?  That way, rather than having
cond_resched do any work, you have the fqs logic recognize that a
particular CPU has gone too long without quiescing, without disturbing
that CPU at all if it hasn't gone too long.

- Josh Triplett
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