From mboxrd@z Thu Jan 1 00:00:00 1970 From: Grant Edwards Subject: Re: Using patch-2.6.33.7.2-rt30 increases latency and CPU usage? Date: Thu, 10 May 2012 08:53:20 -0500 Message-ID: <20120510135320.GA9568@grante> References: <4FAAEBF3.10907@netacquire.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: linux-rt-users@vger.kernel.org To: Remy Bohmer Return-path: Received: from mail-qa0-f49.google.com ([209.85.216.49]:63742 "EHLO mail-qa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754122Ab2EJNx0 (ORCPT ); Thu, 10 May 2012 09:53:26 -0400 Received: by qabj40 with SMTP id j40so637420qab.1 for ; Thu, 10 May 2012 06:53:25 -0700 (PDT) Content-Disposition: inline In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Thu, May 10, 2012 at 11:46:26AM +0200, Remy Bohmer wrote: > 2012/5/10 Grant Edwards : > >> Unfortunately, the requirements are a bit fuzzy -- I've got an ISR >> deadline of about 20us that I'm trying to meet [I wouldn't mind a >> little chat with the person who designed _that_ requirement into the >> hardware]. =A0What I don't know is how hard that deadline is. =A0Wit= h the >> RT patch (and without IRQF_NODELAY), I miss the deadline most of the >> time (I'd guess about 80% of the time). >=20 > Given my experience with these cores and the RT patch: This 20usec is > too close. That's pretty much what I've decided. > There are some places in the code that have interrupt disable section > of about 30usec (also process ctx-switch cache flush have high > impact) > > So, I expect you will not get this very robust. > > If the 20usec requirement is hard, you can also look at using an FIQ. Does the normal AT91 kernel not use FIQs at all? If so, I might be able to dedicate the FIQ to this function and have it happen without the Linux kernel knowing about it (or affecting it). Any communication between it and a normal user-task would have to be handled carefully... --=20 Grant -- To unsubscribe from this list: send the line "unsubscribe linux-rt-user= s" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html