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=-13.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 4CC80C43331 for ; Tue, 31 Mar 2020 15:09:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1A39F20838 for ; Tue, 31 Mar 2020 15:09:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="NiYS9jB1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730708AbgCaPJS (ORCPT ); Tue, 31 Mar 2020 11:09:18 -0400 Received: from mail-qk1-f195.google.com ([209.85.222.195]:44255 "EHLO mail-qk1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726526AbgCaPJR (ORCPT ); Tue, 31 Mar 2020 11:09:17 -0400 Received: by mail-qk1-f195.google.com with SMTP id j4so23272785qkc.11 for ; Tue, 31 Mar 2020 08:09:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=1I3V/oT2mqbiXfqRekZesope51/Mfc/oSX4BHUhE4J4=; b=NiYS9jB1Px4PsqvHN1UGdxUhc5aSF0dRIMU3OzkGIN2lsaRvtJNeeGS/lCoijGJD8c HUEvtIRZ4FM+K9NhlEOgcVyaM6o2slWFdcweJX+44idvOFod8i4WlkgkgdujA5HOJYYm S3ZN2CWBk+MB57dWvvkZaoL7JXthswf/sVWKM= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=1I3V/oT2mqbiXfqRekZesope51/Mfc/oSX4BHUhE4J4=; b=GuLR79PIXtdvNsJGDfQsS6XE6EeLjlKnqHnVR4bP3CXSjYpD4FmdbRph+qMEV7K8Q0 EYV8C8vJ4Va+7z9O8+W2z/Q8CBGuYgl7hliXASLdPXbkuLqRKnZ9Jr/LC2AK5GQ/eTsa 7j+DzjkjgZ7PBFWXKuNNNrjssLXQcgCyCrkTP4XrPdCfF4Kn81fG8DWdYL3g/R5vX1v/ e5nOC+T4PWxqicL6V69ffNzaK0lJudnDIqMGG1XLeqBTC9ofwuG/Nv9ip4VkhdRP+tiW Ztd6bblOSYbA84sKwGoS+rs8sfCJwaI2n3ZswVuFU5QLaEu8M/dtNnHnm1OjhMITqSts 0+2w== X-Gm-Message-State: ANhLgQ0DQOomv3yn7Qi+KHl0qNpN41glDNT2Yq0/qG+QUGbxegXZkDfD R+Xbt0zgLzLgIhuGhZ4Ib/uXs7XBvgo= X-Google-Smtp-Source: ADFU+vvwg3fjNiVxq1OHVz/lt26BshlSMLdLRgOKuq1vbH2mIwdbY4yegCT9+EC0/Y4lx84auhOHAQ== X-Received: by 2002:a37:a93:: with SMTP id 141mr5443291qkk.244.1585667352335; Tue, 31 Mar 2020 08:09:12 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id g6sm14323892qtc.31.2020.03.31.08.09.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2020 08:09:11 -0700 (PDT) Date: Tue, 31 Mar 2020 11:09:11 -0400 From: Joel Fernandes To: Uladzislau Rezki Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org, peterz@infradead.org, neilb@suse.com, vbabka@suse.cz, mgorman@suse.de, Andrew Morton , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , Steven Rostedt Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern Message-ID: <20200331150911.GC236678@google.com> References: <20200331131628.153118-1-joel@joelfernandes.org> <20200331140433.GA26498@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200331140433.GA26498@pc636> User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 31, 2020 at 04:04:33PM +0200, Uladzislau Rezki wrote: > On Tue, Mar 31, 2020 at 09:16:28AM -0400, Joel Fernandes (Google) wrote: > > In kfree_rcu() headless implementation (where the caller need not pass > > an rcu_head, but rather directly pass a pointer to an object), we have > > a fall-back where we allocate a rcu_head wrapper for the caller (not the > > common case). This brings the pattern of needing to allocate some memory > > to free some memory. Currently we use GFP_ATOMIC flag to try harder for > > this allocation, however the GFP_MEMALLOC flag is more tailored to this > > pattern. We need to try harder not only during atomic context, but also > > during non-atomic context anyway. So use the GFP_MEMALLOC flag instead. > > > > Also remove the __GFP_NOWARN flag simply because although we do have a > > synchronize_rcu() fallback for absolutely worst case, we still would > > like to not enter that path and atleast trigger a warning to the user. > > > > Cc: linux-mm@kvack.org > > Cc: rcu@vger.kernel.org > > Cc: willy@infradead.org > > Cc: peterz@infradead.org > > Cc: neilb@suse.com > > Cc: vbabka@suse.cz > > Cc: mgorman@suse.de > > Cc: Andrew Morton > > Signed-off-by: Joel Fernandes (Google) > > --- > > > > This patch is based on the (not yet upstream) code in: > > git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree) > > > > It is a follow-up to the posted series: > > https://lore.kernel.org/lkml/20200330023248.164994-1-joel@joelfernandes.org/ > > > > > > kernel/rcu/tree.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 4be763355c9fb..965deefffdd58 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj) > > > > if (!ptr) > > ptr = kmalloc(sizeof(unsigned long *) + > > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN); > > + sizeof(struct rcu_head), GFP_MEMALLOC); > > > Hello, Joel > > I have some questions regarding improving it, see below them: > > Do you mean __GFP_MEMALLOC? Can that flag be used in atomic context? > Actually we do allocate there under spin lock. Should be combined with > GFP_ATOMIC | __GFP_MEMALLOC? Yes, I mean __GFP_MEMALLOC. Sorry, the patch was just to show the idea and marked as RFC. Good point on the atomic aspect of this path, you are right we cannot sleep. I believe the GFP_NOWAIT I mentioned in my last reply will take care of that? > As for removing __GFP_NOWARN. Actually it is expectable that an > allocation can fail, if so we follow last emergency case. You > can see the trace but what would you do with that information? Yes, the benefit of the trace/warning is that the user can switch to a non-headless API and avoid the synchronize_rcu(), that would help them get faster kfree_rcu() performance instead of having silent slowdowns. It also tells us whether the headless API is worth it in the long run, I think it is worth it because we will likely never hit the synchronize_rcu() failsafe. But if we hit it a lot, at least it wont happen silently. Paul was concerned about following scenario with hitting synchronize_rcu(): 1. Consider a system under memory pressure. 2. Consider some other subsystem X depending on another system Y which uses kfree_rcu(). If Y doesn't complete the operation in time, X accumulates more memory. 3. Since kfree_rcu() on Y hits synchronize_rcu() a lot, it slows it down. This causes X to further allocate memory, further causing a chain reaction. Paul, please correct me if I'm wrong. thanks, - Joel