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 X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 95BF0C43603 for ; Tue, 17 Dec 2019 14:01:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 644F620716 for ; Tue, 17 Dec 2019 14:01:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="WPbOKcS1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728683AbfLQOBI (ORCPT ); Tue, 17 Dec 2019 09:01:08 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:28239 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728573AbfLQOBI (ORCPT ); Tue, 17 Dec 2019 09:01:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1576591266; 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=uwJQ1dZUryn91uRJBuAlpM0T0gwPgs8B9ULHpt/Vuc4=; b=WPbOKcS1dMp7hmcD6Lxx00C6LL1W7dH4qUNELlyFt8JF4fzzUqsShRRj1aUXowgy4TSLb4 jlX7epOL/+psCJ39qd9I1EfLnInAkmASuCNrkyo9/X1ZEULUSIhFrBPYblD4tfQbn5KAFM HqiHITN+dfMp9VRQ4AOkgUp5tQ8xVZk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-283-nQmLEYpTOua68PthYW3UWA-1; Tue, 17 Dec 2019 09:01:03 -0500 X-MC-Unique: nQmLEYpTOua68PthYW3UWA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7E413107ACC5; Tue, 17 Dec 2019 14:01:01 +0000 (UTC) Received: from llong.remote.csb (ovpn-123-81.rdu2.redhat.com [10.10.123.81]) by smtp.corp.redhat.com (Postfix) with ESMTP id 04E2360FC2; Tue, 17 Dec 2019 14:00:59 +0000 (UTC) Subject: Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in non-task context To: Kirill Tkhai , Michal Hocko Cc: Mike Kravetz , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , Davidlohr Bueso , Andi Kleen , "Aneesh Kumar K.V" References: <20191217012508.31495-1-longman@redhat.com> <20191217093143.GC31063@dhcp22.suse.cz> <87c2ff49-999e-3196-791f-36e3d42ad79c@virtuozzo.com> From: Waiman Long Organization: Red Hat Message-ID: <0b8a59a0-517f-1387-ad00-cb47fb5fc50c@redhat.com> Date: Tue, 17 Dec 2019 09:00:59 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <87c2ff49-999e-3196-791f-36e3d42ad79c@virtuozzo.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/17/19 5:50 AM, Kirill Tkhai wrote: > On 17.12.2019 12:31, Michal Hocko wrote: >> On Mon 16-12-19 20:25:08, Waiman Long wrote: >> [...] >>> Both the hugetbl_lock and the subpool lock can be acquired in >>> free_huge_page(). One way to solve the problem is to make both locks >>> irq-safe. >> Please document why we do not take this, quite natural path and instead >> we have to come up with an elaborate way instead. I believe the primary >> motivation is that some operations under those locks are quite >> expensive. Please add that to the changelog and ideally to the code as >> well. We probably want to fix those anyway and then this would be a >> temporary workaround. >> >>> Another alternative is to defer the freeing to a workqueue job. >>> >>> This patch implements the deferred freeing by adding a >>> free_hpage_workfn() work function to do the actual freeing. The >>> free_huge_page() call in a non-task context saves the page to be freed >>> in the hpage_freelist linked list in a lockless manner. >> Do we need to over complicate this (presumably) rare event by a lockless >> algorithm? Why cannot we use a dedicated spin lock for for the linked >> list manipulation? This should be really a trivial code without an >> additional burden of all the lockless subtleties. > Why not llist_add()/llist_del_all() ? > The llist_add() and llist_del_all() are just simple helpers. Because this lockless case involve synchronization of two variables, the llist helpers do not directly apply here. So the rests cannot be used. It will look awkward it is partially converted to use the helpers. If we convert to use a lock as suggested by Michal, using the helpers will be an overkill as xchg() will not be needed. Cheers, Longman