From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id CF989280252 for ; Tue, 27 Sep 2016 05:42:55 -0400 (EDT) Received: by mail-wm0-f70.google.com with SMTP id w84so1656191wmg.1 for ; Tue, 27 Sep 2016 02:42:55 -0700 (PDT) Received: from outbound-smtp09.blacknight.com (outbound-smtp09.blacknight.com. [46.22.139.14]) by mx.google.com with ESMTPS id s9si1497798wjv.17.2016.09.27.02.42.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Sep 2016 02:42:54 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp09.blacknight.com (Postfix) with ESMTPS id 243EF1C18B2 for ; Tue, 27 Sep 2016 10:42:54 +0100 (IST) Date: Tue, 27 Sep 2016 10:42:49 +0100 From: Mel Gorman Subject: Re: page_waitqueue() considered harmful Message-ID: <20160927094249.GA3903@techsingularity.net> References: <20160927073055.GM2794@worktop> <20160927085412.GD2838@techsingularity.net> <20160927091117.GA23640@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20160927091117.GA23640@node.shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Peter Zijlstra , Linus Torvalds , Andrew Morton , "Kirill A. Shutemov" , Johannes Weiner , Jan Kara , Rik van Riel , linux-mm On Tue, Sep 27, 2016 at 12:11:17PM +0300, Kirill A. Shutemov wrote: > On Tue, Sep 27, 2016 at 09:54:12AM +0100, Mel Gorman wrote: > > On Tue, Sep 27, 2016 at 09:30:55AM +0200, Peter Zijlstra wrote: > > > > Also, if those bitlock ops had a different bit that showed contention, > > > > we could actually skip *all* of this, and just see that "oh, nobody is > > > > waiting on this page anyway, so there's no point in looking up those > > > > wait queues". We don't have that many "__wait_on_bit()" users, maybe > > > > we could say that the bitlocks do have to haev *two* bits: one for the > > > > lock bit itself, and one for "there is contention". > > > > > > That would be fairly simple to implement, the difficulty would be > > > actually getting a page-flag to use for this. We're running pretty low > > > in available bits :/ > > > > Simple is relative unless I drastically overcomplicated things and it > > wouldn't be the first time. 64-bit only side-steps the page flag issue > > as long as we can live with that. > > Looks like we don't ever lock slab pages. Unless I miss something. > I don't think we do but direct PageSlab checks might be problematic if it was a false-positive due to a locked page and we'd have to be very careful about any races due to two bits being used. While we shouldn't rule it out, I think it's important to first look at that original patch and see if it's remotely acceptable and makes enough difference to a real workload to matter. If so, then we could consider additional complexity on top to make it work on 32-bit -- maybe separated by one release as it took a long time to flush out subtle bugs with the PG_waiters approach. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org