From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762129Ab3JQBMA (ORCPT ); Wed, 16 Oct 2013 21:12:00 -0400 Received: from mail-pa0-f46.google.com ([209.85.220.46]:61549 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761829Ab3JQBL6 (ORCPT ); Wed, 16 Oct 2013 21:11:58 -0400 Date: Wed, 16 Oct 2013 18:11:56 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Mel Gorman cc: Andrew Morton , Linux-MM , LKML Subject: Re: [PATCH] mm: Do not walk all of system memory during show_mem In-Reply-To: <20131016104228.GM11028@suse.de> Message-ID: References: <20131016104228.GM11028@suse.de> User-Agent: Alpine 2.02 (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 Wed, 16 Oct 2013, Mel Gorman wrote: > It has been reported on very large machines that show_mem is taking almost > 5 minutes to display information. This is a serious problem if there is > an OOM storm. The bulk of the cost is in show_mem doing a very expensive > PFN walk to give us the following information > > Total RAM: Also available as totalram_pages > Highmem pages: Also available as totalhigh_pages > Reserved pages: Can be inferred from the zone structure > Shared pages: PFN walk required > Unshared pages: PFN walk required > Quick pages: Per-cpu walk required > > Only the shared/unshared pages requires a full PFN walk but that information > is useless. It is also inaccurate as page pins of unshared pages would > be accounted for as shared. Even if the information was accurate, I'm > struggling to think how the shared/unshared information could be useful > for debugging OOM conditions. Maybe it was useful before rmap existed when > reclaiming shared pages was costly but it is less relevant today. > > The PFN walk could be optimised a bit but why bother as the information is > useless. This patch deletes the PFN walker and infers the total RAM, highmem > and reserved pages count from struct zone. It omits the shared/unshared page > usage on the grounds that it is useless. It also corrects the reporting > of HighMem as HighMem/MovableOnly as ZONE_MOVABLE has similar problems to > HighMem with respect to lowmem/highmem exhaustion. > We haven't been hit by this for the oom killer, but we did get hit with this for page allocation failure warnings as a result of having irqs disabled and passing GFP_ATOMIC to the page allocator without GFP_NOWARN. That was the intention of passing SHOW_MEM_FILTER_PAGE_COUNT into show_mem() in 4b59e6c47309 ("mm, show_mem: suppress page counts in non-blockable contexts"). With this, I assume we can just remove SHOW_MEM_FILTER_PAGE_COUNT entirely?