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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 ECC40C433FF for ; Tue, 30 Jul 2019 12:32:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BF8F52089E for ; Tue, 30 Jul 2019 12:32:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564489960; bh=fBUqCoenMUXj+UTof67hBbwUfxH9olno/QLTQe6JHw0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=akyX2jDmhVLZdMYOzWRX7K5TFZh4/S6gAWSt/qqtj3KrEdVnPmbsmfczvDAmV/YDr nYgLnGo8q7l+/L6QokzlLPxijfaCNN+q45cA1y1sm3Q223DJFP7BK6AeE0BBy7iO4t HIaPsCd9qh3NLerpUspvvaGniZWYdbCNBnz4BVXY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729694AbfG3Mcj (ORCPT ); Tue, 30 Jul 2019 08:32:39 -0400 Received: from mx2.suse.de ([195.135.220.15]:55110 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726986AbfG3Mcj (ORCPT ); Tue, 30 Jul 2019 08:32:39 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EDE1AAFD2; Tue, 30 Jul 2019 12:32:37 +0000 (UTC) Date: Tue, 30 Jul 2019 14:32:37 +0200 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , LKML , linux-mm , Miguel de Dios , Wei Wang , Johannes Weiner , Mel Gorman Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range Message-ID: <20190730123237.GR9330@dhcp22.suse.cz> References: <20190729071037.241581-1-minchan@kernel.org> <20190729074523.GC9330@dhcp22.suse.cz> <20190729082052.GA258885@google.com> <20190729083515.GD9330@dhcp22.suse.cz> <20190730121110.GA184615@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190730121110.GA184615@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 30-07-19 21:11:10, Minchan Kim wrote: > On Mon, Jul 29, 2019 at 10:35:15AM +0200, Michal Hocko wrote: > > On Mon 29-07-19 17:20:52, Minchan Kim wrote: > > > On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote: > > > > On Mon 29-07-19 16:10:37, Minchan Kim wrote: > > > > > In our testing(carmera recording), Miguel and Wei found unmap_page_range > > > > > takes above 6ms with preemption disabled easily. When I see that, the > > > > > reason is it holds page table spinlock during entire 512 page operation > > > > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't > > > > > run in the time because it could make frame drop or glitch audio problem. > > > > > > > > Where is the time spent during the tear down? 512 pages doesn't sound > > > > like a lot to tear down. Is it the TLB flushing? > > > > > > Miguel confirmed there is no such big latency without mark_page_accessed > > > in zap_pte_range so I guess it's the contention of LRU lock as well as > > > heavy activate_page overhead which is not trivial, either. > > > > Please give us more details ideally with some numbers. > > I had a time to benchmark it via adding some trace_printk hooks between > pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing > device is 2018 premium mobile device. > > I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the > task runs on little core even though it doesn't have any IPI and LRU > lock contention. It's already too heavy. > > If I remove activate_page, 35-40% overhead of zap_pte_range is gone > so most of overhead(about 0.7ms) comes from activate_page via > mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could > accumulate up to several ms. Thanks for this information. This is something that should be a part of the changelog. I am sorry to still poke into this because I still do not have a full understanding of what is going on and while I do not object to drop the spinlock I still suspect this is papering over a deeper problem. If mark_page_accessed is really expensive then why do we even bother to do it in the tear down path in the first place? Why don't we simply set a referenced bit on the page to reflect the young pte bit? I might be missing something here of course. -- Michal Hocko SUSE Labs