linux-kernel - Re: [PATCH tip/core/rcu] Reduce overhead of cond

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Date:	Mon, 23 Jun 2014 10:42:17 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, dave.hansen@...el.com,
	cl@...two.org, umgwanakikbuti@...il.com
Subject: Re: [PATCH tip/core/rcu] Reduce overhead of cond_resched() checks
 for RCU

On Mon, Jun 23, 2014 at 10:19:05AM -0700, Andi Kleen wrote:
> > In 3.10, RCU had 14,046 lines of code, not counting documentation and
> > test scripting.  In 3.15, RCU had 13,208 lines of code, again not counting
> > documentation and test scripting.  That is a decrease of almost 1KLoC,
> > so your wish is granted.
> 
> Ok that's good progress.

Glad you like it!

> > CONFIG_RCU_NOCB_CPU, CONFIG_RCU_NOCB_CPU_NONE, CONFIG_RCU_NOCB_CPU_ZERO,
> > and CONFIG_RCU_NOCB_CPU_ALL.  It also might be reasonable to replace
> > uses of CONFIG_PROVE_RCU with CONFIG_PROVE_LOCKING, thus allowing
> > CONFIG_PROVE_RCU to be eliminated.  CONFIG_PROVE_RCU_DELAY hasn't proven
> > very good at finding bugs, so I am considering eliminating it as well.
> > Given recent and planned changes related to RCU's stall-warning stack
> > dumping, I hope to eliminate both CONFIG_RCU_CPU_STALL_VERBOSE and
> > CONFIG_RCU_CPU_STALL_INFO, making them both happen unconditionally.
> > (And yes, I should probably make CONFIG_RCU_CPU_STALL_INFO be the default
> > for some time beforehand.)  I have also been considering getting rid of
> > CONFIG_RCU_FANOUT_EXACT, given that it appears that no one uses it.
> 
> Yes please to all.
> 
> Sounds good thanks.

Very good!

Please note that this will take some time.  For example, getting rid
of CONFIG_RCU_CPU_STALL_TIMEOUT resulted in a series of bugs over a
period of a well over a year.  It turned out that very few people were
exercising it while it was non-default.  Hopefully, Fengguang Wu's
RANDCONFIG testing is testing these things more these days.

Also, some of them might have non-obvious effects on performance,
witness the cond_resched() fun and excitement.

							Thanx, Paul

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