From nobody Thu May 14 08:25:47 2026 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 B4772C433EF for ; Sat, 16 Apr 2022 01:07:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229557AbiDPBKT (ORCPT ); Fri, 15 Apr 2022 21:10:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38626 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229436AbiDPBKS (ORCPT ); Fri, 15 Apr 2022 21:10:18 -0400 Received: from out1.migadu.com (out1.migadu.com [91.121.223.63]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8B39516665B for ; Fri, 15 Apr 2022 18:07:42 -0700 (PDT) X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1650069678; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=C5nkTrq4xMgYIDmhUhVhC2Dq7TG9dRaDSUT0wVRb144=; b=Jo2/gZ05dZmfC/MFjPD+Ds9RazbpGZ3I7R9PcnlMSTwT7x2TY0Hg3IsG8oiZff1gW75ZYv tfWe7WPDHcAf/j2xhHdyc+Jg5hRoAz9OgqZlsX81+XLBSBBRonAbTz29hjw51prfNIvNSN ei8ecQgEloMgkibAiWOgzN6iNK2LOsA= From: Roman Gushchin To: linux-mm@kvack.org Cc: Andrew Morton , Dave Chinner , linux-kernel@vger.kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Yang Shi , Roman Gushchin Subject: [PATCH] mm: do not call add_nr_deferred() with zero deferred Date: Fri, 15 Apr 2022 17:41:04 -0700 Message-Id: <20220416004104.4089743-1-roman.gushchin@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" add_nr_deferred() is often called with next_deferred equal to 0. For instance, it's happening under low memory pressure for any shrinkers with a low number of cached objects. A corresponding trace looks like: <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \ super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ unused scan count 0 new scan count 0 total_scan 0 \ last shrinker return val 0 <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \ super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ unused scan count 0 new scan count 0 total_scan 0 \ last shrinker return val 0 <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \ super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \ scan count 0 new scan count 0 total_scan 0 \ last shrinker return val 0 This lead to unnecessary checks and atomic operations, which can be avoided by checking next_deferred for not being zero before calling add_nr_deferred(). In this case the mm_shrink_slab_end trace point will get a potentially slightly outdated "new scan count" value, but it's totally fine. Signed-off-by: Roman Gushchin Acked-by: David Hildenbrand --- mm/vmscan.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index d4a7d2bd276d..19d3d4fa1aad 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_cont= rol *shrinkctl, * move the unused scan count back into the shrinker in a * manner that handles concurrent updates. */ - new_nr =3D add_nr_deferred(next_deferred, shrinker, shrinkctl); + if (next_deferred) + new_nr =3D add_nr_deferred(next_deferred, shrinker, shrinkctl); + else + new_nr =3D nr; =20 trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, tot= al_scan); return freed; --=20 2.35.1