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 000E5C433F5 for ; Tue, 19 Apr 2022 06:34:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348867AbiDSGgn (ORCPT ); Tue, 19 Apr 2022 02:36:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47334 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235053AbiDSGgk (ORCPT ); Tue, 19 Apr 2022 02:36:40 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EEA042C64A for ; Mon, 18 Apr 2022 23:33:58 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id A4AD6B81186 for ; Tue, 19 Apr 2022 06:33:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BAB14C385A5; Tue, 19 Apr 2022 06:33:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1650350036; bh=4/35g5b4ZPqhbhpRVje281c0PbxC2UdS5Qu7e3PjXDg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=UuI5vRWx9SIIgxgR49VZO41fWcEu/6+vg7zcF43WXML0/Dfp4hhvIrCoUtscu7j4c 6vdOBRuKqH6RQ+16CdG5HWRuX4obTn1x5iNxxNBzVtH97c/RrYd5GNKKGQZ4AHStku hj+0G4NmXx5vzTIBu+Q8hGyALhuPv8RSHBK0tAqnZs+rwL/JZckpBjXHG6XSMu+BBF D29OEOurKDKtGwLHiLKPEaZDl4yhRnhFBOXNBsOsRMZ8SHxbNVWQ/aXIFrJLzamo8k omevuMhJxmq8P2YDqePLhx0fWFVmLXSPhW6l4DeMK3Dwocyg3f3JxOEZsoUHWY5SK6 V17w6KG6EJ0pQ== Date: Tue, 19 Apr 2022 09:33:48 +0300 From: Mike Rapoport To: Roman Gushchin Cc: linux-mm@kvack.org, Andrew Morton , Dave Chinner , linux-kernel@vger.kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Yang Shi Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface Message-ID: References: <20220416002756.4087977-1-roman.gushchin@linux.dev> 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 Mon, Apr 18, 2022 at 10:27:34AM -0700, Roman Gushchin wrote: > On Mon, Apr 18, 2022 at 12:27:36PM +0300, Mike Rapoport wrote: > > On Fri, Apr 15, 2022 at 05:27:51PM -0700, Roman Gushchin wrote: > > > There are 50+ different shrinkers in the kernel, many with their own bells and > > > whistles. Under the memory pressure the kernel applies some pressure on each of > > > them in the order of which they were created/registered in the system. Some > > > of them can contain only few objects, some can be quite large. Some can be > > > effective at reclaiming memory, some not. > > > > > > The only existing debugging mechanism is a couple of tracepoints in > > > do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end. They aren't > > > covering everything though: shrinkers which report 0 objects will never show up, > > > there is no support for memcg-aware shrinkers. Shrinkers are identified by their > > > scan function, which is not always enough (e.g. hard to guess which super > > > block's shrinker it is having only "super_cache_scan"). They are a passive > > > mechanism: there is no way to call into counting and scanning of an individual > > > shrinker and profile it. > > > > > > To provide a better visibility and debug options for memory shrinkers > > > this patchset introduces a /sys/kernel/shrinker interface, to some extent > > > similar to /sys/kernel/slab. > > > > Wouldn't debugfs better fit the purpose of shrinker debugging? > > I think sysfs fits better, but not a very strong opinion. > > Even though the interface is likely not very useful for the general > public, big cloud instances might wanna enable it to gather statistics > (and it's certainly what we gonna do at Facebook) and to provide > additional data when something is off. They might not have debugfs > mounted. And it's really similar to /sys/kernel/slab. And there is also similar /proc/vmallocinfo so why not /proc/shrinker? ;-) I suspect slab ended up in sysfs because nobody suggested to use debugfs back then. I've been able to track the transition from /proc/slabinfo to /proc/slubinfo to /sys/kernel/slab, but could not find why Christoph chose sysfs in the end. > Are there any reasons why debugfs is preferable? debugfs is more flexible because it's not stable kernel ABI so if there will be need/desire to change the layout and content of the files with debugfs it can be done more easily. Is this a real problem for Facebook to mount debugfs? ;-) > Thanks! -- Sincerely yours, Mike.