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=-5.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 D91E1C433E6 for ; Thu, 28 Jan 2021 15:17:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 96EB264DD6 for ; Thu, 28 Jan 2021 15:17:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231286AbhA1PRv (ORCPT ); Thu, 28 Jan 2021 10:17:51 -0500 Received: from mx2.suse.de ([195.135.220.15]:59002 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231171AbhA1PRt (ORCPT ); Thu, 28 Jan 2021 10:17:49 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1611847022; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=CUUsqoDshqZ4qwQ1Kc4iSyaByclB2HlvDzwBh7gRls4=; b=ioVcpdleXlKYLmDeMvdOTbmadVIiiTiPMNjAXN3IgvJ0IvirC8P2v31xUobENq54J/wb2w PSX/PY9AtEbCb/sE6Twy/kOgmVmfXfdyJS0I/19RceFU/kk6U+ypGZKvQiMKjZb33mV/iX iyZsKoaaBfeEp0ndRGvraK+yuGh7Ks8= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id BF9A2AE44; Thu, 28 Jan 2021 15:17:02 +0000 (UTC) Date: Thu, 28 Jan 2021 16:17:01 +0100 From: Michal Hocko To: Uladzislau Rezki Cc: 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: 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: <20210128151152.GA1867@pc638.lan> Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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. -- Michal Hocko SUSE Labs