From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Date: Fri, 09 Nov 2007 10:59:51 +0000 Subject: Re: conflict between tickless and perfmon2 Message-Id: <1194605991.6289.162.camel@twins> List-Id: References: <20071109104438.GB18909@frankl.hpl.hp.com> In-Reply-To: <20071109104438.GB18909@frankl.hpl.hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: eranian@hpl.hp.com Cc: perfmon2-devel@lists.sourceforge.net, perfmon@napali.hpl.hp.com, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, Thomas Gleixner , Ingo Molnar On Fri, 2007-11-09 at 02:44 -0800, Stephane Eranian wrote: > Hello, > > We have identified a conflict between TICKLESS (CONFIG_NO_HZ) and > the current perfmon2 implementation. The problem impacts system-wide > sessions using timeout-based event set multiplexing. > > Event set multiplexing allows monitoring tools to measure more events > than there are actual performance counters on the processor. Events > are grouped in sets which are then multiplexed onto the actual counters. > Switching can be triggered either by a timeout or by a counter overflow. > This is supported for per-thread and system-wide sessions. > > For timeout-based switching, the duration expressed in nanoseconds is > meant to represent wall-clock time in system-wide mode, and execution > time in per-thread mode. Granularity is limited by HZ. > > The current implementation for timeout is a simple hook on the timer > interrupt path in apic_*.c:smp_local_timer_interrupt(). Unfortunately, > this does not work when tickless is enabled: we get much less set > switches than expected on an idle system. > > It looks like a solution would be to change the implementation of > timeout-based switching to use HR timers instead. Similar to what is > done for ITIMER_REAL and ITIMER_VIRTUAL. > > Unless someone has a better proposal, I will experiment with this on > 2.6.24-rc2. Might help if you CC the tickless folks :-)