linux-kernel - Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140704050541.GL4603@linux.vnet.ibm.com>
Date:	Thu, 3 Jul 2014 22:05:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread
 wakeups

On Fri, Jul 04, 2014 at 05:23:56AM +0200, Mike Galbraith wrote:
> On Thu, 2014-07-03 at 09:29 -0700, Paul E. McKenney wrote: 
> > On Thu, Jul 03, 2014 at 07:48:40AM +0200, Mike Galbraith wrote:
> > > On Wed, 2014-07-02 at 22:21 -0700, Paul E. McKenney wrote: 
> > > > On Thu, Jul 03, 2014 at 05:31:19AM +0200, Mike Galbraith wrote:
> > > 
> > > > > NO_HZ_FULL is a property of a set of CPUs.  isolcpus is supposed to go
> > > > > away as being a redundant interface to manage a single property of a set
> > > > > of CPUs, but it's perfectly fine for NO_HZ_FULL to add an interface to
> > > > > manage a single property of a set of CPUs.  What am I missing? 
> > > > 
> > > > Well, for now, it can only be specified at build time or at boot time.
> > > > In theory, it is possible to change a CPU from being callback-offloaded
> > > > to not at runtime, but there would need to be an extremely good reason
> > > > for adding that level of complexity.  Lots of "fun" races in there...
> > > 
> > > Yeah, understood.
> > > 
> > > (still it's a NO_HZ_FULL wart though IMHO, would be prettier and more
> > > usable if it eventually became unified with cpuset and learned how to
> > > tap-dance properly;)
> > 
> > Agreed, it would in some sense be nice.  What specifically do you need
> > it for?
> 
> I personally have zero use for the thing (git/vi aren't particularly
> perturbation sensitive;). I'm just doing occasional drive-by testing
> from a distro perspective, how well does it work, what does it cost etc.
> 
> >   Are you really running workloads that generate large numbers of
> > callbacks spread across most of the CPUs?  It was this sort of workload
> > that caused Rik's system to show scary CPU-time accumulation, due to
> > the high overhead of frequent one-to-many wakeups.
> > 
> > If your systems aren't running that kind of high-callback-rate workload,
> > just set CONFIG_RCU_NOCB_CPU_ALL=y and don't worry about it.
> > 
> > If your systems -are- running that kind of high-callback-rate workload,
> > but your system has fewer than 200 CPUs, ensure that you have enough
> > housekeeping CPUs to allow the grace-period kthread sufficient CPU time,
> > set CONFIG_RCU_NOCB_CPU_ALL=y and don't worry about it.
> > 
> > If your systems -are- running that kind of high-callback-rate workload,
> > and your system has more than 200 CPUs, apply the following patch,
> > set CONFIG_RCU_NOCB_CPU_ALL=y and once again don't worry about it.  ;-)
> 
> Turn it on and don't worry about it is exactly what distros want the
> obscure feature with very few users to be.  Last time I did a drive-by,
> my boxen said I should continue to worry about it ;-)

Yep, which is the reason for the patch on the last email.

Then again, exactly which feature and which reason for worry?

							Thanx, Paul

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/