From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id D57566B004D for ; Wed, 30 Sep 2009 21:36:47 -0400 (EDT) Date: Thu, 1 Oct 2009 04:02:07 +0200 From: Nick Piggin Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked() Message-ID: <20091001020207.GL6327@wotan.suse.de> References: <20090926031537.GA10176@localhost> <20090926034936.GK30185@one.firstfloor.org> <20090926105259.GA5496@localhost> <20090926113156.GA12240@localhost> <20090927104739.GA1666@localhost> <20090927192025.GA6327@wotan.suse.de> <20090928084401.GA22131@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090928084401.GA22131@localhost> Sender: owner-linux-mm@kvack.org To: Wu Fengguang Cc: Andi Kleen , Andrew Morton , Hugh Dickins , "linux-mm@kvack.org" , LKML List-ID: On Mon, Sep 28, 2009 at 04:44:01PM +0800, Wu Fengguang wrote: > On Mon, Sep 28, 2009 at 03:20:25AM +0800, Nick Piggin wrote: > > On Sun, Sep 27, 2009 at 06:47:39PM +0800, Wu Fengguang wrote: > > > > > > > > And standard deviation is 0.04%, much larger than the difference 0.008% .. > > > > > > Sorry that's not correct. I improved the accounting by treating > > > function0+function1 from two CPUs as an integral entity: > > > > > > total time add_to_page_cache_lru percent stddev > > > before 3880166848.722 9683329.610 0.250% 0.014% > > > after 3828516894.376 9778088.870 0.256% 0.012% > > > delta 0.006% > > > > I don't understand why you're doing this NFS workload to measure? > > Because it is the first convenient workload hit my mind, and avoids > real disk IO :) Using tmpfs or sparse files is probably a lot easier. > > I see significant nfs, networking protocol and device overheads in > > your profiles, also you're hitting some locks or something which > > is causing massive context switching. So I don't think this is a > > good test. > > Yes there are overheads. However it is a real and common workload. Right, but so are lots of other workloads that don't hit add_to_page_cache heavily :) > > But anyway as Hugh points out, you need to compare with a > > *completely* fixed kernel, which includes auditing all users of page > > flags non-atomically (slab, notably, but possibly also other > > places). > > That's good point. We can do more benchmarks when more fixes are > available. However I suspect their design goal will be "fix them > without introducing noticeable overheads" :) s/noticeable// The problem with all the non-noticeable overheads that we're continually adding to the kernel is that we're adding them to the kernel. Non-noticeable part only makes it worse because you can't bisect them :) > > One other thing to keep in mind that I will mention is that I am > > going to push in a patch to the page allocator to allow callers > > to avoid the refcounting (atomic_dec_and_test) in page lifetime, > > which is especially important for SLUB and takes more cycles off > > the page allocator... > > > > I don't know exactly what you're going to do after that to get a > > stable reference to slab pages. I guess you can read the page > > flags and speculatively take some slab locks and recheck etc... > > For reliably we could skip page lock on zero refcounted pages. > > We may lose the PG_hwpoison bit on races with __SetPageSlub*, however > it should be an acceptable imperfection. I think if you're wiling to accept these problems, then it is completely reasonable to also accept similar races with kernel fastpaths to avoid extra overheads there. -- 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