Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757054AbXFVVNu (ORCPT ); Fri, 22 Jun 2007 17:13:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751849AbXFVVNm (ORCPT ); Fri, 22 Jun 2007 17:13:42 -0400 Received: from www.osadl.org ([213.239.205.134]:37846 "EHLO mail.tglx.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751773AbXFVVNm (ORCPT ); Fri, 22 Jun 2007 17:13:42 -0400 Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues From: Thomas Gleixner To: Christoph Hellwig Cc: Ingo Molnar , Linus Torvalds , Steven Rostedt , LKML , Andrew Morton , john stultz , Oleg Nesterov , "Paul E. McKenney" , Dipankar Sarma , "David S. Miller" , matthew.wilcox@hp.com, kuznet@ms2.inr.ac.ru In-Reply-To: <20070622210013.GA1353@infradead.org> References: <20070622040014.234651401@goodmis.org> <20070622204058.GA11777@elte.hu> <20070622210013.GA1353@infradead.org> Content-Type: text/plain Date: Fri, 22 Jun 2007 23:13:30 +0200 Message-Id: <1182546810.20203.122.camel@chaos> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 (2.10.1-4.fc7) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1774 Lines: 41 On Fri, 2007-06-22 at 22:00 +0100, Christoph Hellwig wrote: > Note that we also have a lot of inefficiency in the way we do deferred > processing. Think of a setup where you run a XFS filesystem runs over > a megaraid adapter. > > (1) we get a real hardirq, which just clears the interrupt and then > deferes to a tasklet > (2) tasklet walks the producer / consumer queue and then calls scsi_done > for each completeted scsi command which ends up doing > raise_softirq_irqoff(BLOCK_SOFTIRQ); > (3) block softirq does the heavy lifting for command completion and finally > calls back into the bio's completion routine > (4) xfs wants to avoid irq safe locking and thus deferes the command to a > kthread > > This is rather inefficient due to all the (semi-)context switches already > and not by far the worst setup given that a lot of dm modules can involve > another thread in the process. > > Now if just plain convert tasklets to a thread based abstraction this > existing code becomes really dumb because we go from hardirq to process > context to go back to softirq context to go back to process context. > > Ouch! > > I think we need to put a little more though into how we can optimize our > irq path for the full stack. Using irqthreads in an intelligent way might > be one option, but we'll need a lot of heavy benchmarking whatever way > we go. Your above scenario screams for a threaded interrupt handler, where you actually can unify a lot of this into one single context. tglx - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/