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Date: Thu, 5 Mar 2015 08:16:11 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jason Low <jason.low2@hp.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        Scott Norton <scott.norton@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability
Message-ID: <20150305161611.GE5773@linux.vnet.ibm.com>
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On Thu, Mar 05, 2015 at 05:00:05PM +0100, Frederic Weisbecker wrote:
> On Thu, Mar 05, 2015 at 07:56:59AM -0800, Paul E. McKenney wrote:
> > On Thu, Mar 05, 2015 at 04:35:09PM +0100, Frederic Weisbecker wrote:
> > > So, in the case we are calling that right after setting cputimer->running, I guess we are fine
> > > because we just updated cputimer with the freshest values.
> > > 
> > > But if we are reading this a while after, say several ticks further, there is a chance that
> > > we read stale values since we don't lock anymore.
> > > 
> > > I don't know if it matters or not, I guess it depends how stale it can be and how much precision
> > > we expect from posix cpu timers. It probably doesn't matter.
> > > 
> > > But just in case, atomic64_read_return(&cputimer->utime, 0) would make sure we get the freshest
> > > value because it performs a full barrier, at the cost of more overhead of course.
> > 
> > Well, if we are running within a guest OS, we might be delayed at any point
> > for quite some time.  Even with interrupts disabled.
> 
> You mean delayed because of the overhead of atomic_add_return() or the stale value
> of cptimer-> fields? 

Because of preemption of the guest OS's VCPUs by the host OS.

								Thanx, Paul