From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [PATCH] qemu-kvm: Consolidate kvm_eat_signals Date: Mon, 12 May 2008 09:31:46 -0500 Message-ID: <482854D2.50100@codemonkey.ws> References: <482820DC.6030509@web.de> <48282B16.7050006@qumranet.com> <48282D13.9060903@web.de> <48283071.5030600@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel , Jan Kiszka To: Avi Kivity Return-path: In-Reply-To: <48283071.5030600@qumranet.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces@lists.sourceforge.net Errors-To: kvm-devel-bounces@lists.sourceforge.net List-Id: kvm.vger.kernel.org Avi Kivity wrote: > Jan Kiszka wrote: > >>> Given that with the iothread we spend very little time processing >>> signals in vcpu threads, maybe it's better to drop the loop completely. >>> The common case is zero or one pending signals. The uncommon case of >>> two or more pending signals will be handled by the KVM_RUN ioctl >>> returning immediately with -EINTR (i.e. in the outer loop). >>> >>> >>> >> You mean >> >> static void kvm_main_loop_wait(CPUState *env, int timeout) >> { >> pthread_mutex_unlock(&qemu_mutex); >> kvm_eat_signal(env, timeout); >> pthread_mutex_lock(&qemu_mutex); >> cpu_single_env = env; >> >> vcpu_info[env->cpu_index].signalled = 0; >> } >> >> ? >> >> > > Yes. The loop was a (perhaps premature) optimization that is now > totally unnecessary, unless I'm missing something quite large. > It used to be that kvm_eat_signal() selected after consuming as many signals as possible while only sleeping once. That's why there's a combination of sleeping and polling. Now the VCPU threads never select so the whole loop can be simplified to a single sigtimedwait() that always blocks. In reality, I don't think sigtimedwait() is really needed/useful for VCPUs anymore. We only use it to catch SIG_IPI and we only use SIG_IPI to break out of sleeping. I don't see any reason why we couldn't switch over to using a file descriptor for notification (or a pthread condition). In the very least, we could just select() on nothing and allow SIG_IPI to break us out of the select. Regards, Anthony Liguori > Oh. There used to be a bug where we didn't check for a pending signal > before the first guest entry, so this would add a lot of latency > (effectively making the bug window much larger). That was only closed > in 2.6.24 (by 7e66f350). > > ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone