From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751547AbcA1FIn (ORCPT ); Thu, 28 Jan 2016 00:08:43 -0500 Received: from LGEAMRELO11.lge.com ([156.147.23.51]:46461 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080AbcA1FIk (ORCPT ); Thu, 28 Jan 2016 00:08:40 -0500 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: iamjoonsoo.kim@lge.com X-Original-SENDERIP: 10.177.222.138 X-Original-MAILFROM: iamjoonsoo.kim@lge.com Date: Thu, 28 Jan 2016 14:08:44 +0900 From: Joonsoo Kim To: David Rientjes Cc: Christoph Lameter , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value Message-ID: <20160128050844.GD14467@js1304-P5Q-DELUXE> References: <1448346123-2699-1-git-send-email-iamjoonsoo.kim@lge.com> <20151125025735.GC9563@js1304-P5Q-DELUXE> <20151126015252.GA13138@js1304-P5Q-DELUXE> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 On Wed, Jan 27, 2016 at 03:13:12PM -0800, David Rientjes wrote: > 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? Just a small uncomfortable thing is that calculation is done with different kinds of metric. For example, comparing vmstat values (NR_INACTIVE_*, NR_ACTIVE_*) with per-zone atomic NR_ISOLATED_* looks ugly and error-prone because their accuracy is different. Thanks.