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=-4.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 02A97C433DB for ; Thu, 28 Jan 2021 15:32:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B937A60235 for ; Thu, 28 Jan 2021 15:32:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231174AbhA1Pb4 (ORCPT ); Thu, 28 Jan 2021 10:31:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56220 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231438AbhA1PbC (ORCPT ); Thu, 28 Jan 2021 10:31:02 -0500 Received: from mail-lf1-x12f.google.com (mail-lf1-x12f.google.com [IPv6:2a00:1450:4864:20::12f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2B6C0C061573; Thu, 28 Jan 2021 07:30:22 -0800 (PST) Received: by mail-lf1-x12f.google.com with SMTP id p21so8105968lfu.11; Thu, 28 Jan 2021 07:30:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=R+Hue0PWlTxdyWc5K6RSHcz9rExWoyGU+yGHd9hzTV0=; b=aOVKWLC/LNskFli8f/Sqb0ejCNpPOgvj4L+dRMZXBxLUD2ua5XcEr4YAs93zwtIhbp JK1172wcLyUJ1HAUgQcsSUqZtoAZANorPdQRa6eoIGGew3NI3ZVOBQQVbY8waDUhhRmA itMIsLiSp2JWeMazv5MczitXTQhUnEaoJA1ckuFeopYgdr1/dZ7DWwMnv20twYfuj+NC nQxDFNsLhnT++nhT+qLCxbgCsUocjYVRgTFxoEPx7sNM7VNBLnsq2bVGB6zcKOaLuHej XyQ2sqsyM8/UDlRu2AOmsv7QaCZtdZkiC1bSkoEOdkVLJWAuJakE798fAiXwsHHENJrC 3y+A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:date:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=R+Hue0PWlTxdyWc5K6RSHcz9rExWoyGU+yGHd9hzTV0=; b=X62FrZBZ9Y9+/T2rJKARWIgidWJGoAOzyXTnmdVIPw+pyIBgv6kHdsdG5qoIqB0A/+ ZT+fCl7j6xd5BPnNIYmwPTqiM/NDt1tvrqtDqHqghHc/OaG5m14HPa5mo/aVAE8m0FLD +1dguAy2y5BqlDErNXGynNaneAzq6NJ4KUm6FB2vP2a2Dl702vbAN6rcE3L0+2i6FSQL e6JNII3PQPfQtquwOBdgkWAbtl1bjgzYr/un10wOKfW+VT+IHuf9yj+btN2fBLahJb/7 +ALbBlN/FVL565PnHC/DvJFxgvRybu5C2XpBj+LOESQbA3D3O1jTFvAGgxxCyvC84704 yAnA== X-Gm-Message-State: AOAM533Fib1FmWtlzxepvko2p6rLwWGTpCfvKACyUZAhgBcrPb1WxcKF x91+WCbDhqCCZSRud6UtuzI= X-Google-Smtp-Source: ABdhPJy56uEiInFhFpbpttAzsRdD+R+o4+uVFk6y67VZEtJzVAKTz0VYUpxH6QPZcvuXH5mmH84K9g== X-Received: by 2002:a05:6512:510:: with SMTP id o16mr7817099lfb.378.1611847820573; Thu, 28 Jan 2021 07:30:20 -0800 (PST) Received: from pc638.lan (h5ef52e3d.seluork.dyn.perspektivbredband.net. [94.245.46.61]) by smtp.gmail.com with ESMTPSA id t22sm689273lfk.128.2021.01.28.07.30.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Jan 2021 07:30:19 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 28 Jan 2021 16:30:17 +0100 To: Michal Hocko Cc: Uladzislau Rezki , LKML , RCU , "Paul E . McKenney" , Michael Ellerman , Andrew Morton , Daniel Axtens , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Peter Zijlstra , Thomas Gleixner , "Theodore Y . Ts'o" , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument Message-ID: <20210128153017.GA2006@pc638.lan> References: <20210120162148.1973-1-urezki@gmail.com> <20210125132236.GJ827@dhcp22.suse.cz> <20210125143150.GA2282@pc638.lan> <20210125153943.GN827@dhcp22.suse.cz> <20210125162559.GA52712@pc638.lan> <20210128151152.GA1867@pc638.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Jan 28, 2021 at 04:17:01PM +0100, Michal Hocko wrote: > On Thu 28-01-21 16:11:52, Uladzislau Rezki wrote: > > On Mon, Jan 25, 2021 at 05:25:59PM +0100, Uladzislau Rezki wrote: > > > On Mon, Jan 25, 2021 at 04:39:43PM +0100, Michal Hocko wrote: > > > > On Mon 25-01-21 15:31:50, Uladzislau Rezki wrote: > > > > > > On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote: > > > > > > > For a single argument we can directly request a page from a caller > > > > > > > context when a "carry page block" is run out of free spots. Instead > > > > > > > of hitting a slow path we can request an extra page by demand and > > > > > > > proceed with a fast path. > > > > > > > > > > > > > > A single-argument kvfree_rcu() must be invoked in sleepable contexts, > > > > > > > and that its fallback is the relatively high latency synchronize_rcu(). > > > > > > > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL > > > > > > > to allow limited sleeping within the memory allocator. > > > > > > > > > > > > __GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy > > > > > > way to allocate without triggering the OOM killer. Is this really what > > > > > > you need/want? Is __GFP_NORETRY too weak? > > > > > > > > > > > Hm... We agreed to proceed with limited lightwait memory direct reclaim. > > > > > Johannes Weiner proposed to go with __GFP_NORETRY flag as a starting > > > > > point: https://www.spinics.net/lists/rcu/msg02856.html > > > > > > > > > > > > > > > So I'm inclined to suggest __GFP_NORETRY as a starting point, and make > > > > > further decisions based on instrumentation of the success rates of > > > > > these opportunistic allocations. > > > > > > > > > > > > > I completely agree with Johannes here. > > > > > > > > > but for some reason, i can't find a tail or head of it, we introduced > > > > > __GFP_RETRY_MAYFAIL what is a heavy one from a time consuming point of view. > > > > > What we would like to avoid. > > > > > > > > Not that I object to this use but I think it would be much better to use > > > > it based on actual data. Going along with it right away might become a > > > > future burden to make any changes in this aspect later on due to lack of > > > > exact reasoning. General rule of thumb for __GFP_RETRY_MAYFAIL is really > > > > try as hard as it can get without being really disruptive (like OOM > > > > killing something). And your wording didn't really give me that > > > > impression. > > > > > > > Initially i proposed just to go with GFP_NOWAIT flag. But later on there > > > was a discussion about a fallback path, that uses synchronize_rcu() can be > > > slow, thus minimizing its hitting would be great. So, here we go with a > > > trade off. > > > > > > Doing it hard as __GFP_RETRY_MAYFAIL can do, is not worth(IMHO), but to have some > > > light-wait requests would be acceptable. That is why __GFP_NORETRY was proposed. > > > > > > There were simple criterias we discussed which we would like to achieve: > > > > > > a) minimize a fallback hitting; > > > b) avoid of OOM involving; > > > c) avoid of dipping into the emergency reserves. See kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument kvfree_rcu() > > > > > One question here. Since the code that triggers a page request can be > > directly invoked from reclaim context as well as outside of it. We had > > a concern about if any recursion is possible, but what i see it is safe. > > The context that does it can not enter it twice: > > > > > > /* Avoid recursion of direct reclaim */ > > if (current->flags & PF_MEMALLOC) > > goto nopage; > > > > Yes this is a recursion protection. > > > What about any deadlocking in regards to below following flags? > > > > GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN > > and __GFP_NOMEMALLOC will make sure that the allocation will not consume > all the memory reserves. The later should be clarified in one of your > patches I have acked IIRC. > Yep, it is clarified and reflected in another patch you ACKed. Thanks! -- Vlad Rezki