Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754393AbbGWWyv (ORCPT ); Thu, 23 Jul 2015 18:54:51 -0400 Received: from mail-pa0-f47.google.com ([209.85.220.47]:34572 "EHLO mail-pa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754066AbbGWWyr (ORCPT ); Thu, 23 Jul 2015 18:54:47 -0400 Date: Thu, 23 Jul 2015 15:54:43 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: =?UTF-8?Q?J=C3=B6rn_Engel?= cc: Spencer Baugh , Andrew Morton , Naoya Horiguchi , Davidlohr Bueso , Mike Kravetz , Luiz Capitulino , "open list:MEMORY MANAGEMENT" , open list , Spencer Baugh , Joern Engel Subject: Re: [PATCH] hugetlb: cond_resched for set_max_huge_pages and follow_hugetlb_page In-Reply-To: <20150723223651.GH24876@Sligo.logfs.org> Message-ID: References: <1437688476-3399-1-git-send-email-sbaugh@catern.com> <20150723223651.GH24876@Sligo.logfs.org> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="397176738-215516084-1437692085=:7871" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2351 Lines: 59 This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --397176738-215516084-1437692085=:7871 Content-Type: TEXT/PLAIN; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT On Thu, 23 Jul 2015, J?rn Engel wrote: > > This is wrong, you'd want to do any cond_resched() before the page > > allocation to avoid racing with an update to h->nr_huge_pages or > > h->surplus_huge_pages while hugetlb_lock was dropped that would result in > > the page having been uselessly allocated. > > There are three options. Either > /* some allocation */ > cond_resched(); > or > cond_resched(); > /* some allocation */ > or > if (cond_resched()) { > spin_lock(&hugetlb_lock); > continue; > } > /* some allocation */ > > I think you want the second option instead of the first. That way we > have a little less memory allocation for the time we are scheduled out. > Sure, we can do that. It probably doesn't make a big difference either > way, but why not. > The loop is dropping the lock simply to do the allocation and it needs to compare with the user-written number of hugepages to allocate. What we don't want is to allocate, reschedule, and check if we really needed to allocate. That's what your patch does because it races with persistent_huge_page(). It's probably the worst place to do it. Rather, what you want to do is check if you need to allocate, reschedule if needed (and if so, recheck), and then allocate. > If you are asking for the third option, I would rather avoid that. It > makes the code more complex and doesn't change the fact that we have a > race and better be able to handle the race. The code size growth will > likely cost us more performance that we would ever gain. nr_huge_pages > tends to get updated once per system boot. > Your third option is nonsensical, you didn't save the state of whether you locked the lock so you can't reliably unlock it, and you cannot hold a spinlock while allocating in this context. --397176738-215516084-1437692085=:7871-- -- 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/