From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [Question] Hooks for scheduler tracing (CFS) Date: Thu, 26 Jul 2007 11:02:26 -0400 Message-ID: <20070726150226.GB1299@Krystal> References: <3efb10970707161246se06ab22i32872cfe6fa4f2f6@mail.gmail.com> <1184615557.2698.3.camel@laptopd505.fenrus.org> <20070726072858.GC13061@in.ibm.com> <20070726133138.GA30559@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: Ankita Garg , Arjan van de Ven , linux@bohmer.net, LKML , RT-Users , Ingo Molnar To: "Frank Ch. Eigler" Return-path: Received: from tomts36-srv.bellnexxia.net ([209.226.175.93]:53885 "EHLO tomts36-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755719AbXGZPCa (ORCPT ); Thu, 26 Jul 2007 11:02:30 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org * Frank Ch. Eigler (fche@redhat.com) wrote: > Mathieu Desnoyers writes: > > > [...] > > The problem is also in _stp_print_flush, not *only* in relay code: > > void _stp_print_flush (void) > > ... > > spin_lock(&_stp_print_lock); > > ... > > spin_unlock(&_stp_print_lock); > > > > Those will turn into mutexes with -rt. > > Indeed, plus systemtap-generated locking code uses rwlocks, > local_irq_save/restore or preempt_disable, in various places. Could > someone point to a place that spells out what would be more > appropriate way of ensuring atomicity while being compatible with -rt? > > - FChE AFAIK, for your needs either: - Use atomic ops - Use per-cpu data with preempt disabling/irq disabling - Use the original "real" spin locks/rwlocks (raw_*). - Don't play with timers or wakeups, since this kernel code uses the "standard" spin locks (sleepable in -rt). You just don't want to sleep in the tracing code. Make sure that the sub-buffer switch code respects that too: it is the most tricky and yet less executed part of the tracing code, so it's easy for bugs to slip there and yet be undetected for a while. Since you will likely disable preemption, make sure your tracing code executes in a deterministic time. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68