From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3EC24C433F5 for ; Thu, 19 May 2022 12:14:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237809AbiESMOR (ORCPT ); Thu, 19 May 2022 08:14:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39972 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237748AbiESMNt (ORCPT ); Thu, 19 May 2022 08:13:49 -0400 Received: from outbound-smtp55.blacknight.com (outbound-smtp55.blacknight.com [46.22.136.239]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0B7C17A833 for ; Thu, 19 May 2022 05:13:45 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp55.blacknight.com (Postfix) with ESMTPS id 5B084FAD39 for ; Thu, 19 May 2022 13:13:44 +0100 (IST) Received: (qmail 28743 invoked from network); 19 May 2022 12:13:44 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.198.246]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 19 May 2022 12:13:44 -0000 Date: Thu, 19 May 2022 13:13:41 +0100 From: Mel Gorman To: Vlastimil Babka Cc: Andrew Morton , Nicolas Saenz Julienne , Marcelo Tosatti , Michal Hocko , LKML , Linux-MM Subject: Re: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue Message-ID: <20220519121341.GV3441@techsingularity.net> References: <20220512085043.5234-1-mgorman@techsingularity.net> <20220512085043.5234-5-mgorman@techsingularity.net> <5a0c9b7c-3620-3e0f-7510-d0fc3fa6ceda@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <5a0c9b7c-3620-3e0f-7510-d0fc3fa6ceda@suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 19, 2022 at 12:57:01PM +0200, Vlastimil Babka wrote: > On 5/12/22 10:50, Mel Gorman wrote: > > The VM_BUG_ON check for a valid page can be avoided with a simple > > change in the flow. The ZONE_BOOSTED_WATERMARK is unlikely in general > > and even more unlikely if the page allocation failed so mark the > > branch unlikely. > > Hm, so that makes a DEBUG_VM config avoid the check. On the other hand, > it puts it on the path returning from rmqueue_pcplist() for all configs, > and that should be the fast path. So unless things further change in the > following patches, it doesn't seem that useful? > You're right -- the fast path ends up with both a if (page) and if (!page) checks. Andrew, can you drop the patch mm-page_alloc-remove-unnecessary-page-==-null-check-in-rmqueue.patch from your tree please? Originally the flow was important when I was writing the patch and later became unnecessary. However, it reminded me of another problem I thought of when writing this and then forgotten to note it in the changelog. If the page allocation fails then ZONE_BOOSTED_WATERMARK should still be tested and cleared before waking kswapd. It could happen if an allocation attempt tried to fallback to another migratetype and still fail to find a suitable page. This is true whether going through the PCP lists or not. So what do you think of me adding this patch to a follow-up series? --8<-- mm/page_alloc: Remove mistaken page == NULL check in rmqueue If a page allocation fails, the ZONE_BOOSTER_WATERMARK should be tested, cleared and kswapd woken whether the allocation attempt was via the PCP or directly via the buddy list. Remove the page == NULL so the ZONE_BOOSTED_WATERMARK bit is checked unconditionally. As it is unlikely that ZONE_BOOSTED_WATERMARK is set, mark the branch accordingly. Signed-off-by: Mel Gorman --- mm/page_alloc.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 1c4c54503a5d..61d5bc2efffe 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3765,12 +3765,10 @@ struct page *rmqueue(struct zone *preferred_zone, page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags, migratetype); - if (unlikely(!page)) - return NULL; out: /* Separate test+clear to avoid unnecessary atomics */ - if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) { + if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) { clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags); wakeup_kswapd(zone, 0, 0, zone_idx(zone)); }