Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755117Ab2JKUGO (ORCPT ); Thu, 11 Oct 2012 16:06:14 -0400 Received: from gir.skynet.ie ([193.1.99.77]:55459 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753091Ab2JKUGM (ORCPT ); Thu, 11 Oct 2012 16:06:12 -0400 Date: Thu, 11 Oct 2012 21:06:08 +0100 From: Mel Gorman To: Andrea Arcangeli Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linus Torvalds , Andrew Morton , Peter Zijlstra , Ingo Molnar , Hugh Dickins , Rik van Riel , Johannes Weiner , Hillf Danton , Andrew Jones , Dan Smith , Thomas Gleixner , Paul Turner , Christoph Lameter , Suresh Siddha , Mike Galbraith , "Paul E. McKenney" Subject: Re: [PATCH 07/33] autonuma: mm_autonuma and task_autonuma data structures Message-ID: <20121011200608.GO3317@csn.ul.ie> References: <1349308275-2174-1-git-send-email-aarcange@redhat.com> <1349308275-2174-8-git-send-email-aarcange@redhat.com> <20121011122827.GT3317@csn.ul.ie> <20121011171519.GQ1818@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20121011171519.GQ1818@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4107 Lines: 113 On Thu, Oct 11, 2012 at 07:15:20PM +0200, Andrea Arcangeli wrote: > On Thu, Oct 11, 2012 at 01:28:27PM +0100, Mel Gorman wrote: > > s/togehter/together/ > > Fixed. > > > > > > + * knumad_scan structure. > > > + */ > > > +struct mm_autonuma { > > > > Nit but this is very similar in principle to mm_slot for transparent > > huge pages. It might be worth renaming both to mm_thp_slot and > > mm_autonuma_slot to set the expectation they are very similar in nature. > > Could potentially be made generic but probably overkill. > > Agreed. A plain rename to mm_autonuma_slot would have the only cons of > making some code spill over 80 col ;). > Fair enough :) > > > + /* link for knuma_scand's list of mm structures to scan */ > > > + struct list_head mm_node; > > > + /* Pointer to associated mm structure */ > > > + struct mm_struct *mm; > > > + > > > + /* > > > + * Zeroed from here during allocation, check > > > + * mm_autonuma_reset() if you alter the below. > > > + */ > > > + > > > + /* > > > + * Pass counter for this mm. This exist only to be able to > > > + * tell when it's time to apply the exponential backoff on the > > > + * task_autonuma statistics. > > > + */ > > > + unsigned long mm_numa_fault_pass; > > > + /* Total number of pages that will trigger NUMA faults for this mm */ > > > + unsigned long mm_numa_fault_tot; > > > + /* Number of pages that will trigger NUMA faults for each [nid] */ > > > + unsigned long mm_numa_fault[0]; > > > + /* do not add more variables here, the above array size is dynamic */ > > > +}; > > > > How cache hot is this structure? nodes are sharing counters in the same > > cache lines so if updates are frequent this will bounce like a mad yoke. > > Profiles will tell for sure but it's possible that some sort of per-cpu > > hilarity will be necessary here in the future. > > On autonuma27 this is only written by knuma_scand so it won't risk to > bounce. > > On autonuma28 however it's updated by the numa hinting page fault > locklessy and so your concern is very real, and the cacheline bounces > will materialize. It will be related to the knuma_scan thing though so once every 10 seconds, we might see a sudden spike in cache conflicts. Is that accurate? Something like perf top might detect when this happens but it can be inferred using perf probe on the fault handler too. > It'll cause more interconnect traffic before the > workload converges too. I thought about that, but I wanted the > mm_autonuma updated in real time as migration happens otherwise it > converges more slowly if we have to wait until the next pass to bring > mm_autonuma statistical data in sync with the migration > activities. Converging more slowly looked worse than paying more > cacheline bounces. > You could argue that slower converging also means more cross-node traffic so it costs either way. > It's a tradeoff. And if it's not a good one, we can go back to > autonuma27 mm_autonuma stat gathering method and converge slower but > without any cacheline bouncing in the NUMA hinting page faults. At > least it's lockless. > Yep. > > > + unsigned long task_numa_fault_pass; > > > + /* Total number of eligible pages that triggered NUMA faults */ > > > + unsigned long task_numa_fault_tot; > > > + /* Number of pages that triggered NUMA faults for each [nid] */ > > > + unsigned long task_numa_fault[0]; > > > + /* do not add more variables here, the above array size is dynamic */ > > > +}; > > > + > > > > Same question about cache hotness. > > Here it's per-thread, so there won't be risk of accesses interleaved > by different CPUs. > Ok thanks. With that clarification Acked-by: Mel Gorman While I still have concerns about the cache behaviour of this the basic intent of the structure will not change no matter how the problem is addressed. -- Mel Gorman SUSE Labs -- 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/