From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932685AbbFWK4I (ORCPT ); Tue, 23 Jun 2015 06:56:08 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:38379 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753437AbbFWKz7 (ORCPT ); Tue, 23 Jun 2015 06:55:59 -0400 Date: Tue, 23 Jun 2015 12:55:48 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: paulmck@linux.vnet.ibm.com, tj@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org, der.herr@hofr.at, dave@stgolabs.net, riel@redhat.com, viro@ZenIV.linux.org.uk, torvalds@linux-foundation.org Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock Message-ID: <20150623105548.GE18673@twins.programming.kicks-ass.net> References: <20150622121623.291363374@infradead.org> <20150622122256.765619039@infradead.org> <20150622222152.GA4460@redhat.com> <20150623100932.GB3644@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150623100932.GB3644@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 23, 2015 at 12:09:32PM +0200, Peter Zijlstra wrote: > We can of course slap a percpu-rwsem in, but I wonder if there's > anything smarter we can do here. Urgh, we cannot use percpu-rwsem here, because that would require percpu_down_write_trylock(), and I'm not sure we can get around the sync_sched() for that. Now try_stop_cpus(), which requires the down_write_trylock() is used to implement synchronize_sched_expedited(). Using sync_sched() to implement sync_sched_expedited would make me happy, but it does somewhat defeat the purpose. Also, I think _expedited is used too eagerly, look at this: +void dm_sync_table(struct mapped_device *md) +{ + synchronize_srcu(&md->io_barrier); + synchronize_rcu_expedited(); +} sync_srcu() is slow already, why then bother with an sync_rcu_expedited() :/