From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=56583 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OOtzV-0008TC-Oa for qemu-devel@nongnu.org; Wed, 16 Jun 2010 10:53:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OOtzU-0006aU-9I for qemu-devel@nongnu.org; Wed, 16 Jun 2010 10:53:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54412) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OOtzU-0006aM-2g for qemu-devel@nongnu.org; Wed, 16 Jun 2010 10:53:20 -0400 Message-ID: <4C18E52B.9010600@redhat.com> Date: Wed, 16 Jun 2010 16:52:27 +0200 From: Paolo Bonzini MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH V4 2/3] qemu: Generic task offloading framework: threadlets References: <20100616115404.10988.62371.stgit@localhost.localdomain> <20100616115656.10988.96529.stgit@localhost.localdomain> <4C18C4C8.8090901@redhat.com> <20100616142236.GA20052@shareable.org> <4C18DFD7.1090102@redhat.com> <4C18E1E8.3030606@linux.vnet.ibm.com> In-Reply-To: <4C18E1E8.3030606@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Gautham R Shenoy , Qemu-development List , "Aneesh Kumar K.V" , Corentin Chary , Avi Kivity On 06/16/2010 04:38 PM, Anthony Liguori wrote: > On 06/16/2010 09:29 AM, Paolo Bonzini wrote: >> On 06/16/2010 04:22 PM, Jamie Lokier wrote: >>> Paolo Bonzini wrote: >>>> These should be (at least for now) block-obj-$(CONFIG_POSIX). >>>> >>>>> + while (QTAILQ_EMPTY(&(queue->request_list))&& >>>>> + (ret != ETIMEDOUT)) { >>>>> + ret = qemu_cond_timedwait(&(queue->cond), >>>>> + &(queue->lock), 10*100000); >>>>> + } >>>> >>>> Using qemu_cond_timedwait is a hack for not properly broadcasting the >>>> condvar in flush_threadlet_queue. >>> >>> Are you sure? It looks like it also expires idle threads after a >>> fixed amount of idle time. >> >> Unnecessary idle threads are immediately expired as soon as the >> threadlet exits if ncecessary, since here > > If a threadlet is waiting to consume more work, unless we do a > pthread_cancel (I dislike cancellation) it will keep waiting until it > gets more work (which would mean it's not actually idle)... Agreed---no cancellation, please. BTW it's obviously okay with signaling the condition when a threadlet is submitted. But when something affects all queue's workers (flush_threadlet_queue) you want a broadcast and using expiration as a substitute is fishy. >> + queue->idle_threads++; >> + >> +check_exit: >> + if (queue->exit || ((queue->idle_threads > 0) && >> + (queue->cur_threads > queue->min_threads))) { >> + /* We exit the queue or we retain minimum number of threads */ >> + break; >> + } >> >> queue->idle_threads > 0 will always be true (so maybe that should be >> changed into an assertion: "this thread is idle, so there must be idle >> threads"). > > queue->exit could be true though so it's necessary to at least check > that condition. Yes, of course. The correct test should be: if (queue->exit || queue->cur_threads > queue->min_threads) But queue->idle_threads will be > 0 even if coming via the goto (which should be eliminated). Or maybe no. After flushing you still want min_threads threads to run. The correct thing then would be: do { ... assert (queue->idle_threads > 0); if (queue->exit) { /* Threads waiting on the barrier cannot do work. */ queue->idle_threads--; qemu_mutex_unlock(&(queue->lock)); qemu_barrier_wait(&queue->barr); qemu_mutex_lock(&(queue->lock)); queue->idle_threads++; } } while (queue->cur_threads <= queue->min_threads); queue->idle_threads--; queue->cur_threads--; qemu_mutex_unlock(&queue->lock); return NULL; So, if min_threads were changed, broadcasting the condition would be enough to exit unwanted threads one at a time, as soon as it grabs the lock. Paolo