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=-5.8 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 85BB5C433E0 for ; Mon, 25 Jan 2021 16:14:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 47F7D22510 for ; Mon, 25 Jan 2021 16:14:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729993AbhAYQOe (ORCPT ); Mon, 25 Jan 2021 11:14:34 -0500 Received: from mx2.suse.de ([195.135.220.15]:58244 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729862AbhAYQO0 (ORCPT ); Mon, 25 Jan 2021 11:14:26 -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=1611589185; 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=zKS/Is5s9cc0mzPuWsCvSSGfuL7/tBg1PVAllAYqfSU=; b=TDxYHXSkfZ2OFjnCSFKr3cLU1pTXjeRqsX+yoD/GrbAWpwcu/BRne7P8UJJeilurf6S61l jIZ9y62a+zX2hG3utViE7U7uzmd6K61dhRsEZgt+4HXB0X8O1jTnhROVRP78seKgn3eOxI 3SHfPblJ7Slvp9x+xkZcQMwER+1afQk= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 2AD06ADD8; Mon, 25 Jan 2021 15:39:45 +0000 (UTC) Date: Mon, 25 Jan 2021 16:39:43 +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: <20210125153943.GN827@dhcp22.suse.cz> References: <20210120162148.1973-1-urezki@gmail.com> <20210125132236.GJ827@dhcp22.suse.cz> <20210125143150.GA2282@pc638.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210125143150.GA2282@pc638.lan> Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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. -- Michal Hocko SUSE Labs