From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux Date: Fri, 20 Feb 2015 15:57:28 +0100 Message-ID: <54E74B58.90706@redhat.com> References: <1424251955-308-1-git-send-email-bogdan.purcareata@freescale.com> <54E73A6C.9080500@suse.de> <54E740E7.5090806@redhat.com> <54E74A8C.30802@linutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, scottwood@freescale.com, mihai.caraman@freescale.com, Thomas Gleixner To: Sebastian Andrzej Siewior , Alexander Graf , Bogdan Purcareata , linuxppc-dev@lists.ozlabs.org, linux-rt-users@vger.kernel.org Return-path: In-Reply-To: <54E74A8C.30802@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org On 20/02/2015 15:54, Sebastian Andrzej Siewior wrote: > Usually you see "scheduling while atomic" on -RT and convert them to > raw locks if it is appropriate. > > Bogdan wrote in 2/2 that he needs to limit the number of CPUs in oder > not cause a DoS and large latencies in the host. I haven't seen an > answer to my why question. Because if the conversation leads to > large latencies in the host then it does not look right. > > Each host PIC has a rawlock and does mostly just mask/unmask and the > raw lock makes sure the value written is not mixed up due to > preemption. > This hardly increase latencies because the "locked" path is very short. > If this conversation leads to higher latencies then the locked path is > too long and hardly suitable to become a rawlock. Yes, but large latencies just mean the code has to be rewritten (x86 doesn't anymore do event injection in an atomic regions for example). Until it is, using raw_spin_lock is correct. Paolo