linux-kernel - Re: [PATCH 0/3] timer: Improve itimers scalability

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Date:	Thu, 27 Aug 2015 17:09:21 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Hideaki Kimura <hideaki.kimura@....com>,
	Jason Low <jason.low2@...com>, Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 0/3] timer: Improve itimers scalability

On Thu, 27 Aug 2015, Steven Rostedt wrote:
> On Thu, 27 Aug 2015 15:18:49 +0200
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > On Wed, Aug 26, 2015 at 04:45:44PM -0700, Hideaki Kimura wrote:
> > > I totally agree that this is not a perfect solution. If there are 10x more
> > > cores and sockets, just the atomic fetch_add might be too expensive.
> > > 
> > > However, it's comparatively/realistically the best thing we can do without
> > > any drawbacks. We can't magically force all library developers to write the
> > > most scalable code always.
> > > 
> > > My point is: this is a safety net, and a very effective one.
> > 
> > I mean the problem here is that a library uses an unscalable profiling feature,
> > unconditionally as soon as you load it without even initializing anything. And
> > this library is used in production.
> > 
> > At first sight, fixing that in the kernel is only a hack that just reduces a bit
> > the symptoms.
> > 
> > What is the technical issue that prevents from fixing that in the library itself?
> > Posix timers can be attached anytime.
> 
> I'm curious to what the downside of this patch set is? If we can fix a
> problem that should be fixed in userspace, but does not harm the kernel
> by doing so, is that bad? (an argument for kdbus? ;-)

The patches are not fixing a problem which should be fixed in user
space. They merily avoid lock contention which happens to be prominent
with that particular library. But avoiding lock contention even for 2
threads is a worthwhile exercise if it does not hurt otherwise. And I
can't see anything what hurts with these patches.

Thanks,

	tglx
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