Date: Thu, 27 Aug 2015 17:17:24 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Hideaki Kimura <hideaki.kimura@hpe.com>, Jason Low <jason.low2@hp.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH 0/3] timer: Improve itimers scalability
Message-ID: <20150827151723.GA22397@lerouge>
References: <20150825202710.d960a928.akpm@linux-foundation.org>
 <1440606804.23728.85.camel@j-VirtualBox>
 <20150826170851.GA5264@redhat.com>
 <1440626847.23728.122.camel@j-VirtualBox>
 <55DE4366.9080104@hpe.com>
 <20150826231326.GE11992@lerouge>
 <55DE4FA8.7050701@hpe.com>
 <20150827131844.GB21105@lerouge>
 <20150827104707.6943c312@gandalf.local.home>
 <alpine.DEB.2.11.1508271707110.15006@nanos>
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On Thu, Aug 27, 2015 at 05:09:21PM +0200, Thomas Gleixner wrote:
> On Thu, 27 Aug 2015, Steven Rostedt wrote:
> > On Thu, 27 Aug 2015 15:18:49 +0200
> > Frederic Weisbecker <fweisbec@gmail.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.

Sure it shouldn't really hurt anyway, since the presense of elapsing timers
itself is checked locklessly.
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