From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Haberler Subject: Re: Detecting PREEMPT_RT Date: Mon, 21 Jan 2013 13:14:13 +0100 Message-ID: <11A0CCD5-A89C-4173-8F94-EB06DFC765FE@mah.priv.at> References: <50FB36A0.7000702@zultron.com> <50FB4BB1.7070005@osadl.org> <1819DC75-5826-4882-A2B7-20953DE3392A@mah.priv.at> <50FC2A33.5010001@osadl.org> <50FC39E4.1090208@zultron.com> <50FC512A.5040909@osadl.org> Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Cc: John Morris , linux-rt-users@vger.kernel.org To: John Kacur Return-path: Received: from ns1.mah.priv.at ([86.59.12.250]:41081 "EHLO sil1.mah.priv.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754364Ab3AUMOT convert rfc822-to-8bit (ORCPT ); Mon, 21 Jan 2013 07:14:19 -0500 In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-ID: John, Am 21.01.2013 um 12:36 schrieb John Kacur: > On Sun, Jan 20, 2013 at 9:18 PM, Carsten Emde wrote: >> John, >> >> >>>>>> Unfortunately, this wiki article is old and unmaintained. You're >>>>>> probably right to be a bit suspicious when checking for the "-rt" >>>>>> suffix >>>>>> of the kernel release (uname -r). But looking for the occurrence of >>>>>> "PREEMPT RT" in the kernel version (uname -v) should work. An >>>>>> application may use the system call uname() and check the version >>>>>> element of the utsname structure. >>>>> >>>>> the utsname.version string match is fine, however: >>>>>> >>>>>> In addition you may want to make sure the system has >>>>>> high-resolution >>>>> >>>>> timers. If so, the timers in /proc/timer_list have ".resolution: 1 >>>>> nsecs". An application may use the function check_timer() from >>>>> cyclictest for this purpose: >>>>>> >>>>>> static int check_timer(void) >>>>>> { >>>>>> struct timespec ts; >>>>>> >>>>>> if (clock_getres(CLOCK_MONOTONIC, &ts)) >>>>>> return 1; >>>>>> >>>>>> return (ts.tv_sec != 0 || ts.tv_nsec != 1); >>>>>> } >>>>> >>>>> >>>>> Just tried this on a generic kernel (2.6.32-45-generic #102-Ubuntu >>>>> SMP Wed Jan 2 21:53:06 UTC 2013 i686 GNU/Linux) and I get ts.tv_sec == 0 >>>>> and ts.tv_nsec == 1, so it looks this cant be used to tell an RT_PREEMPT >>>>> from a vanilla kernel >>>> >>>> I meant boolean AND when I said "In addition". >>> >>> Oh, I interpreted the timer check not as a PREEMPT_RT check, but a >>> separate check for timers, which we must confirm are available before >>> driving heavy machinery. >>> So, if utsname.version contains "PREEMPT RT", then load the preempt-rt >>> module. Once the module is loaded, from an init() function, run sanity >>> checks, including the clock_getres check above. Is that about right? >> >> Yes. Sounds good to me. >> > Note we added a patch, called sysfs-realtime-entry.patch, but I'm not > sure how far back it goes, at least it's in 3.x-rt kernels. > > This is the text from the patch > > "Add a /sys/kernel entry to indicate that the kernel is a > realtime kernel. found this: https://bugzilla.redhat.com/show_bug.cgi?id=728551 and this: https://github.com/clrkwllms/rt-linux/commit/274ca2cb72e013602cebb8dc142a7c69d42f92db looks like it is being used elsewhere: http://softballinfo.net/ret/root/usr/lib/python2.6/site-packages/sos/plugins/kernel_realtime.py > > Clark says that he needs this for udev rules, udev needs to evaluate > if its a PREEMPT_RT kernel a few thousand times and parsing uname > output is too slow or so. > > Are there better solutions? Should it exist and return 0 on !-rt?" > > To use it, you do something like > test -e /sys/kernel/realtime excellent - this works fine for me (I dont see us going back further than 3.4): mah@atom:~$ uname -a Linux atom 3.4.13-rt-preempt-rt22+ #2 SMP PREEMPT RT Fri Oct 19 08:56:20 CEST 2012 i686 GNU/Linux mah@atom:~$ ls -l /sys/kernel/realtime -r--r--r-- 1 root root 4096 2013-01-21 12:53 /sys/kernel/realtime mah@atom:~$ cat /sys/kernel/realtime 1 The way I read it the fingerprint is - '/sys/kernel/realtime' exists and contents is '1' thanks a lot! - Michael > > If you need it in an older kernel that doesn't have it, it should port > trivially. > > Thanks > -- > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html