From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56912) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZIDHU-0001ke-Sa for qemu-devel@nongnu.org; Thu, 23 Jul 2015 05:59:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZIDHQ-0003xd-TY for qemu-devel@nongnu.org; Thu, 23 Jul 2015 05:59:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48559) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZIDHQ-0003xY-Ou for qemu-devel@nongnu.org; Thu, 23 Jul 2015 05:59:08 -0400 References: <1435855010-30882-1-git-send-email-jjherne@linux.vnet.ibm.com> <1435855010-30882-2-git-send-email-jjherne@linux.vnet.ibm.com> <55956A2E.4020806@redhat.com> <55A3CEAF.6030504@linux.vnet.ibm.com> <55A3D5D8.7070902@redhat.com> <55A654D6.5000906@linux.vnet.ibm.com> <55A657ED.3070407@redhat.com> <55A7BDF4.4020509@linux.vnet.ibm.com> From: Paolo Bonzini Message-ID: <55B0BAE6.3040504@redhat.com> Date: Thu, 23 Jul 2015 11:59:02 +0200 MIME-Version: 1.0 In-Reply-To: <55A7BDF4.4020509@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH v4 1/5] cpu: Provide vcpu throttling interface List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: jjherne@linux.vnet.ibm.com, afaerber@suse.de, amit.shah@redhat.com, dgilbert@redhat.com, borntraeger@de.ibm.com, quintela@redhat.com, qemu-devel@nongnu.org On 16/07/2015 16:21, Jason J. Herne wrote: > 1. Using atomic operations to manage throttle_percentage. I'm not sure > where atomics are applicable here. If this is still a concern hopefully > someone can explain. I would use atomic_read/atomic_set in cpu_throttle_set,=20 cpu_throttle_stop, cpu_throttle_active, cpu_throttle_get_percentage. =20 In addition, the function naming seems to be a bit inconsistent: please=20 rename cpu_throttle_set to cpu_throttle_set_percentage. Second, here: >> +static void cpu_throttle_thread(void *opaque) >> +{ >> + double pct =3D (double)throttle_percentage/100; Please use cpu_throttle_get_percentage(), and >> + double throttle_ratio =3D pct / (1 - pct); >> + long sleeptime_ms =3D (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE= ); ... move these computations below the if. I'm also not sure about throttle_ratio, why is it needed? If pct >=3D 0.= 5 you end up with throttle_ratio >=3D 1, i.e. no way for the CPU to do any work= . This would definitely cause a problem with callbacks piling up. >> + if (!throttle_percentage) { >> + return; >> + } >> + >> + qemu_mutex_unlock_iothread(); >> + g_usleep(sleeptime_ms * 1000); /* Convert ms to us for usleep call *= / >> + qemu_mutex_lock_iothread(); >> +} >> + > 2. Callback stacking. And it seems like we are convinced that it is not > a big issue. Anyone disagree? I think it's not a big issue to have many timers, but it is a big issue t= o have many callbacks. What I suggested is this: if (!atomic_xchg(&cpu->throttle_thread_scheduled, 1)) { async_run_on_cpu(cpu, cpu_throttle_thread, NULL); } and in the callback: atomic_set(&cpu->throttle_thread_scheduled, 0); g_usleep(...);=20 Paolo