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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3AE98C433FE for ; Wed, 19 Jan 2022 09:42:51 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 81C3F6B0071; Wed, 19 Jan 2022 04:42:50 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 7CC746B0073; Wed, 19 Jan 2022 04:42:50 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6BB5F6B0074; Wed, 19 Jan 2022 04:42:50 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0070.hostedemail.com [216.40.44.70]) by kanga.kvack.org (Postfix) with ESMTP id 58C4A6B0071 for ; Wed, 19 Jan 2022 04:42:50 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 0B1B7825F89F for ; Wed, 19 Jan 2022 09:42:50 +0000 (UTC) X-FDA: 79046547300.30.4104E6F Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by imf03.hostedemail.com (Postfix) with ESMTP id 8AA1C20008 for ; Wed, 19 Jan 2022 09:42:49 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 6F7542177B; Wed, 19 Jan 2022 09:42:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1642585368; 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=bYZCV4bYim71AB+/QlUD1eg456naM2OEMt3lf0LO1HA=; b=jIi92zpelf8mzF+0yk4eI/jvO3S6FlNCteJxLfwc7T8r1uHCbr7xuG4zEadbUA/uJV4yzZ qUQfgJaXQeUFgTaqS6NtO++fEMmXmUkTiNdIpnj+ThLeu6yvWmkf7ciEjfnvm3GgpYz4Tv 4NRE1rxPOGuRTlwRVhHfBgsSxBQjA0Q= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 83C7EA3B8D; Wed, 19 Jan 2022 09:42:47 +0000 (UTC) Date: Wed, 19 Jan 2022 10:42:47 +0100 From: Michal Hocko To: Yu Zhao Cc: Andrew Morton , Linus Torvalds , Andi Kleen , Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Jesse Barnes , Johannes Weiner , Jonathan Corbet , Matthew Wilcox , Mel Gorman , Michael Larabel , Rik van Riel , Vlastimil Babka , Will Deacon , Ying Huang , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, page-reclaim@google.com, x86@kernel.org, Konstantin Kharlamov Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging Message-ID: References: <20220104202227.2903605-1-yuzhao@google.com> <20220104202227.2903605-7-yuzhao@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 8AA1C20008 X-Stat-Signature: tkejcmokymo989os74n3try61mcuc9co Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=jIi92zpe; spf=pass (imf03.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.28 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-HE-Tag: 1642585369-737735 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 Wed 19-01-22 00:04:10, Yu Zhao wrote: > On Mon, Jan 10, 2022 at 11:54:42AM +0100, Michal Hocko wrote: > > On Sun 09-01-22 21:47:57, Yu Zhao wrote: > > > On Fri, Jan 07, 2022 at 03:44:50PM +0100, Michal Hocko wrote: > > > > On Tue 04-01-22 13:22:25, Yu Zhao wrote: > > > > [...] > > > > > +static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_mm_walk *walk) > > > > > +{ > > > > > + static const struct mm_walk_ops mm_walk_ops = { > > > > > + .test_walk = should_skip_vma, > > > > > + .p4d_entry = walk_pud_range, > > > > > + }; > > > > > + > > > > > + int err; > > > > > +#ifdef CONFIG_MEMCG > > > > > + struct mem_cgroup *memcg = lruvec_memcg(lruvec); > > > > > +#endif > > > > > + > > > > > + walk->next_addr = FIRST_USER_ADDRESS; > > > > > + > > > > > + do { > > > > > + unsigned long start = walk->next_addr; > > > > > + unsigned long end = mm->highest_vm_end; > > > > > + > > > > > + err = -EBUSY; > > > > > + > > > > > + rcu_read_lock(); > > > > > +#ifdef CONFIG_MEMCG > > > > > + if (memcg && atomic_read(&memcg->moving_account)) > > > > > + goto contended; > > > > > +#endif > > > > > + if (!mmap_read_trylock(mm)) > > > > > + goto contended; > > > > > > > > Have you evaluated the behavior under mmap_sem contention? I mean what > > > > would be an effect of some mms being excluded from the walk? This path > > > > is called from direct reclaim and we do allocate with exclusive mmap_sem > > > > IIRC and the trylock can fail in a presence of pending writer if I am > > > > not mistaken so even the read lock holder (e.g. an allocation from the #PF) > > > > can bypass the walk. > > > > > > You are right. Here it must be a trylock; otherwise it can deadlock. > > > > Yeah, this is clear. > > > > > I think there might be a misunderstanding: the aging doesn't > > > exclusively rely on page table walks to gather the accessed bit. It > > > prefers page table walks but it can also fallback to the rmap-based > > > function, i.e., lru_gen_look_around(), which only gathers the accessed > > > bit from at most 64 PTEs and therefore is less efficient. But it still > > > retains about 80% of the performance gains. > > > > I have to say that I really have hard time to understand the runtime > > behavior depending on that interaction. How does the reclaim behave when > > the virtual scan is enabled, partially enabled and almost completely > > disabled due to different constrains? I do not see any such an > > evaluation described in changelogs and I consider this to be a rather > > important information to judge the overall behavior. > > It doesn't have (partially) enabled/disabled states nor does its > behavior change with different reclaim constraints. Having either > would make its design too complex to implement or benchmark. Let me clarify. By "partially enabled" I really meant behavior depedning on runtime conditions. Say mmap_sem cannot be locked for half of scanned tasks and/or allocation for the mm walker fails due to lack of memory. How does this going to affect reclaim efficiency. How does a user/admin know that the memory reclaim is in a "degraded" mode because of the contention? -- Michal Hocko SUSE Labs