From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: get_irq_regs() from soft IRQ Date: Mon, 29 Jun 2009 18:37:57 +0100 Message-ID: <20090629173757.GC9042@n2100.arm.linux.org.uk> References: <200906291631.19562.jpihet@mvista.com> <200906291936.57703.siarhei.siamashka@nokia.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from caramon.arm.linux.org.uk ([78.32.30.218]:44333 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753203AbZF2Rjm (ORCPT ); Mon, 29 Jun 2009 13:39:42 -0400 Content-Disposition: inline In-Reply-To: <200906291936.57703.siarhei.siamashka@nokia.com> Sender: linux-omap-owner@vger.kernel.org List-Id: linux-omap@vger.kernel.org To: Siarhei Siamashka Cc: ext Jean Pihet , "linux-omap@vger.kernel.org" , "linux-arm-kernel@lists.arm.linux.org.uk" , "oprofile-list@lists.sourceforge.net" On Mon, Jun 29, 2009 at 07:36:57PM +0300, Siarhei Siamashka wrote: > On Monday 29 June 2009 17:31:18 ext Jean Pihet wrote: > > I am trying to get the latest IRQ registers from a timer or a work queue > > but I am running into problems: > > - get_irq_regs() returns NULL in some cases, so it is unsuable and even > > causes crash when trying to get the registers values from the returned ptr > > - I never get user space registers, only kernel > > > > The use case is that the performance unit (PMNC) of the Cortex A8 has some > > serious bug, in short the performance counters overflow IRQ is to be > > avoided. The solution I am implementing is to read and reset the counters > > from a work queue that is triggered by a timer. > > Regarding this oprofile related part. I wonder how you can get oprofile > working properly (providing non-bogus results) without performance > counters overflow IRQ generation? I don't think you can - triggering capture on overflow is precisely how oprofile works. The erratum talks about polling for overflow. By doing this, you are in a well defined part of the kernel, which is obviously going to be shown as a hot path for every counter, thus making oprofile useless for kernel work. Deferring the interrupt to a workqueue doesn't resolve the problem either. The problem has nothing to do with what happens after the interrupt occurs - it's about interrupts themselves being lost. I think just accepting that this erratum breaks oprofile is the only realistic solution. ;(