From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755072AbcA0XNQ (ORCPT ); Wed, 27 Jan 2016 18:13:16 -0500 Received: from mail-pa0-f46.google.com ([209.85.220.46]:33096 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754141AbcA0XNO (ORCPT ); Wed, 27 Jan 2016 18:13:14 -0500 Date: Wed, 27 Jan 2016 15:13:12 -0800 (PST) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Joonsoo Kim cc: Christoph Lameter , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value In-Reply-To: <20151126015252.GA13138@js1304-P5Q-DELUXE> Message-ID: References: <1448346123-2699-1-git-send-email-iamjoonsoo.kim@lge.com> <20151125025735.GC9563@js1304-P5Q-DELUXE> <20151126015252.GA13138@js1304-P5Q-DELUXE> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 26 Nov 2015, Joonsoo Kim wrote: > I understand design decision, but, it is better to get value as much > as accurate if there is no performance problem. My patch would not > cause much performance degradation because it is just adding one > this_cpu_read(). > > Consider about following example. Current implementation returns > interesting output if someone do following things. > > v1 = zone_page_state(XXX); > mod_zone_page_state(XXX, 1); > v2 = zone_page_state(XXX); > > v2 would be same with v1 in most of cases even if we already update > it. > > This situation could occurs in page allocation path and others. If > some task try to allocate many pages, then watermark check returns > same values until updating vmstat even if some freepage are allocated. > There are some adjustments for this imprecision but why not do it become > accurate? I think that this change is reasonable trade-off. > I'm not sure that NR_ISOLATED_* should be vmstats in the first place. The most important callers that depend on its accuracy is zone_reclaimable_pages() and the too_many_isolated() loop in both shrink_inactive_list() and memory compaction. If zlc's are updated every 1s, the HZ/10 in those loops don't really matter, they may as well be HZ/2. I think memory compaction updates the counters in the most appropriate way, by incrementing a counter and then finally doing mod_zone_page_state() for the counter. The other updaters are thp collapse and page migration. I discount user-visible vmstats here because the trade-off has already been made that they may be stale for up to 1s and userspace isn't affected. So what happens if we simply convert NR_ISOLATED_* into per-zone atomic64_t?