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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A02CDC433F5 for ; Wed, 19 Jan 2022 09:42:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1352788AbiASJmu (ORCPT ); Wed, 19 Jan 2022 04:42:50 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:59690 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235918AbiASJmt (ORCPT ); Wed, 19 Jan 2022 04:42:49 -0500 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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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