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 DBA12C433FE for ; Tue, 19 Apr 2022 12:56:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344520AbiDSM7G (ORCPT ); Tue, 19 Apr 2022 08:59:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242767AbiDSM7E (ORCPT ); Tue, 19 Apr 2022 08:59:04 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 6B1FA2B192 for ; Tue, 19 Apr 2022 05:56:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1650372981; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=w7CRMFwBAwSytoT7QhMHKmZ9oEqeZXJfZLO3BeitbNk=; b=Ag9wQi3tkWp6uhEatutEtw/H5llf2MK+COvow8cRRrF6cFCg4V1OQeHpCp/G7Y8yFfIZyq 0Mclxts/XKnFGVVjigmYYBMBhHEn66sBwIRohbfpw7SchGSP7+rsXGWj/Ika6XrVU5OUaB 2m1R8Q1GRsAitiVxg9xwfigIwKRVH3Q= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-495-11jULcglOF2YZ-WQSWJQ4Q-1; Tue, 19 Apr 2022 08:56:20 -0400 X-MC-Unique: 11jULcglOF2YZ-WQSWJQ4Q-1 Received: by mail-wm1-f69.google.com with SMTP id y11-20020a7bc18b000000b0038eac019fc0so1183411wmi.9 for ; Tue, 19 Apr 2022 05:56:19 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:organization:in-reply-to :content-transfer-encoding; bh=w7CRMFwBAwSytoT7QhMHKmZ9oEqeZXJfZLO3BeitbNk=; b=qV5TMctCEORNJC/5XCVRkXgJKmVuZbTOWEXQvbzNGa9ZAPiVyp48nx5VJY6zgex1ya TYp1tLf1/bdgGy5wKMxwX5elqdxeO4j8hmSd/dEO/SCX475aYRmcq+1g4AeZ4RRMwY3Q vllvrkgoCbX4IvMo7UfBD9bTAm5JSTI/JFZlTZZWDKeNMeaZZV8a3Ftxmnfhz0cbEsYp iUMO0+yHjgox2NYSfEios6uwXWZGwy8/E5JIcyzKvJgtt8IyuCri6k/jci0zFLET0cX0 iWQWdpYXK8Bdrs/hkJ90zb7+Iky4soBzPb6jM9JbH2FqUdoTH12PJjATL9TnlPUT7QSl to0A== X-Gm-Message-State: AOAM533jizkz+VtmJhgJ2e0zJEkaKc4kwrPoJ42KnVAweGxj8+rZue7N Z+kol0wRTJ6COnqEeA5MAPcKUJrHQ0ZK2/BUdOih2lLYNSxufR0lk1y5Tnf7UXpgkXXPGlOESbI q+cVYfWQuHmfZcXmbL/Fc0DU+ X-Received: by 2002:a5d:6e0d:0:b0:207:a4d9:7950 with SMTP id h13-20020a5d6e0d000000b00207a4d97950mr11552650wrz.477.1650372978817; Tue, 19 Apr 2022 05:56:18 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxYKkTpNyM9dnV8PInAa0R3bc+otnnkjPQ1loj/u9sqAIWt80zuVCGbG64ifL6Knin9j3CtIA== X-Received: by 2002:a5d:6e0d:0:b0:207:a4d9:7950 with SMTP id h13-20020a5d6e0d000000b00207a4d97950mr11552629wrz.477.1650372978524; Tue, 19 Apr 2022 05:56:18 -0700 (PDT) Received: from ?IPV6:2003:cb:c704:5d00:d8c2:fbf6:a608:957a? (p200300cbc7045d00d8c2fbf6a608957a.dip0.t-ipconnect.de. [2003:cb:c704:5d00:d8c2:fbf6:a608:957a]) by smtp.gmail.com with ESMTPSA id g5-20020adfd1e5000000b0020a97e7ba9fsm5533899wrd.92.2022.04.19.05.56.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Apr 2022 05:56:17 -0700 (PDT) Message-ID: <59404249-de0c-c569-d04a-9da38ed14b0a@redhat.com> Date: Tue, 19 Apr 2022 14:56:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred Content-Language: en-US To: Roman Gushchin , linux-mm@kvack.org Cc: Andrew Morton , Dave Chinner , linux-kernel@vger.kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Yang Shi References: <20220416004104.4089743-1-roman.gushchin@linux.dev> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220416004104.4089743-1-roman.gushchin@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16.04.22 02:41, Roman Gushchin wrote: > 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. Sufficient improvement to justify added complexity for anybody reading that code? > > Signed-off-by: Roman Gushchin > --- > 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_control *shrinkctl, > * move the unused scan count back into the shrinker in a > * manner that handles concurrent updates. > */ > - new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > + if (next_deferred) > + new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > + else > + new_nr = nr; > > trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan); > return freed; And if we still want to do this optimization, why not put it into add_nr_deferred()? -- Thanks, David / dhildenb