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=-7.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 DB71AC433E0 for ; Tue, 4 Aug 2020 17:02:48 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id ABA8F207FC for ; Tue, 4 Aug 2020 17:02:48 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ABA8F207FC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 145F38D017A; Tue, 4 Aug 2020 13:02:48 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0F6A18D0081; Tue, 4 Aug 2020 13:02:48 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 00B448D017A; Tue, 4 Aug 2020 13:02:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0159.hostedemail.com [216.40.44.159]) by kanga.kvack.org (Postfix) with ESMTP id DD1048D0081 for ; Tue, 4 Aug 2020 13:02:47 -0400 (EDT) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 7021A181AEF00 for ; Tue, 4 Aug 2020 17:02:47 +0000 (UTC) X-FDA: 77113505574.06.spark01_10007ac26fa8 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin06.hostedemail.com (Postfix) with ESMTP id 9C8EB10039824 for ; Tue, 4 Aug 2020 17:02:40 +0000 (UTC) X-HE-Tag: spark01_10007ac26fa8 X-Filterd-Recvd-Size: 4944 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Tue, 4 Aug 2020 17:02:15 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 22F21B588; Tue, 4 Aug 2020 17:02:30 +0000 (UTC) Subject: Re: [RFC-PROTOTYPE 1/1] mm: Add __GFP_FAST_TRY flag To: "Uladzislau Rezki (Sony)" , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , Matthew Wilcox Cc: "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko References: <20200803163029.1997-1-urezki@gmail.com> From: Vlastimil Babka Message-ID: <1d50a46a-b97f-96b2-8a5c-21075f022f01@suse.cz> Date: Tue, 4 Aug 2020 19:02:14 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200803163029.1997-1-urezki@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 9C8EB10039824 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/3/20 6:30 PM, Uladzislau Rezki (Sony) wrote: > Some background and kfree_rcu() > =============================== > The pointers to be freed are stored in the per-cpu array to improve > performance, to enable an easier-to-use API, to accommodate vmalloc > memmory and to support a single argument of the kfree_rcu() when only > a pointer is passed. More details are below. > > In order to maintain such per-CPU arrays there is a need in dynamic > allocation when a current array is fully populated and a new block is > required. See below the example: > > 0 1 2 3 0 1 2 3 > |p|p|p|p| -> |p|p|p|p| -> NULL > > there are two pointer-blocks, each one can store 4 addresses > which will be freed after a grace period is passed. In reality > we store PAGE_SIZE / sizeof(void *). So what do you actually have without the dynamic allocation, 8 addresses or PAGE_SIZE / sizeof(void *) addresses? And how many dynamically allocated pages did you observe you might need in practice? Can it be somehow quantified the benefit that you are able to allocate up to X pages dynamically from the pcplists, vs a fixed number of pages held just for that purpose + fallback? ... > A number of pre-fetched elements seems does not depend on amount of the > physical memory in a system. In my case it is 63 pages. This step is not It may depend, if you tune vm.percpu_pagelist_fraction sysctl. But I wouldn't know the exact formulas immediately. See pageset_set_high_and_batch(). In any case for your purpose the 'high' value (in e.g. /proc/zoneinfo) is more relevant (it means the maximum pages you might find cached) for you than the 'batch' (how much is cached in one refill). > lock-less. It uses spinlock_t for accessing to the body's zone. This > step is fully covered in the rmqueue_bulk() function. > > Summarizing. The __GFP_FAST_TRY covers only [1] and can not do step [2], > due to the fact that [2] acquires spinlock_t. It implies that it is super > fast, but a higher rate of fails is also expected. > > Usage: __get_free_page(__GFP_FAST_TRY); > > 2) There was a proposal from Matthew Wilcox: https://lkml.org/lkml/2020/7/31/1015 > > > On non-RT, we could make that lock a raw spinlock. On RT, we could > decline to take the lock. We'd need to abstract the spin_lock() away > behind zone_lock(zone), but that should be OK. > > > It would be great to use any existing flag, say GFP_NOWAIT. Suppose we > decline to take the lock across the page allocator for RT. But there is > at least one path that does it outside of the page allocator. GFP_NOWAIT > can wakeup the kswapd, whereas a "wake-up path" uses sleepable lock: > > wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait). > > Probably it can be fixed by the excluding of waking of the kswapd process > defining something like below: Is something missing here? > what is equal to zero and i am not sure if __get_free_page(0) handles > all that correctly, though it allocates and seems working on my test > machine! Please note it is related to "if we can reuse existing flags". > > In the meantime, please see below for a patch that adds a __GFP_FAST_TRY, > which can at least serve as a baseline against which other proposals can > be compared. The patch is based on the 5.8.0-rc3. > > Please RFC. At first glance __GFP_FAST_TRY (more descriptive name? __GFP_NO_LOCKS?) seems better than doing weird things with GFP_NOWAIT, but depends on the real benefits (hence my first questions). Thanks, Vlastimil