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 42461C71153 for ; Tue, 29 Aug 2023 15:18:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237076AbjH2PRq (ORCPT ); Tue, 29 Aug 2023 11:17:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52368 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237213AbjH2PR3 (ORCPT ); Tue, 29 Aug 2023 11:17:29 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F2E9F194; Tue, 29 Aug 2023 08:17:25 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id AD4CE21863; Tue, 29 Aug 2023 15:17:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1693322244; 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=BOigw10DFMaZ6FlTz/kuzcWWW1zJctEYt2mbUACfshw=; b=cf6cfut5qMHHLRMDbwkQ2gYlsH8BstjRGecp7d4wrj6kl1juJGA88uZVLY/nIkvn4joG6o yc1WtUrbFGH37RngP2icfs7mhlOiLLHfvX3nXod/VcciRd/wn+9pFBlSTbq9Qw6JFxynhg vLQnqh8YI4Amh1yo0orGgcpcoRZ6h+E= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 9863913301; Tue, 29 Aug 2023 15:17:24 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 0DXgJAQM7mSfHAAAMHmgww (envelope-from ); Tue, 29 Aug 2023 15:17:24 +0000 Date: Tue, 29 Aug 2023 17:17:24 +0200 From: Michal Hocko To: Waiman Long Cc: Yosry Ahmed , Shakeel Butt , Andrew Morton , Johannes Weiner , Roman Gushchin , Muchun Song , Ivan Babrou , Tejun Heo , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads Message-ID: References: <599b167c-deaf-4b92-aa8b-5767b8608483@redhat.com> <5906501e-4dff-4c66-7ab3-e9193d312270@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5906501e-4dff-4c66-7ab3-e9193d312270@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 29-08-23 11:05:28, Waiman Long wrote: > On 8/29/23 03:27, Michal Hocko wrote: > > On Mon 28-08-23 13:27:23, Waiman Long wrote: > > > On 8/28/23 13:07, Yosry Ahmed wrote: > > > > > Here I agree with you. Let's go with the approach which is easy to > > > > > undo for now. Though I prefer the new explicit interface for flushing, > > > > > that step would be very hard to undo. Let's reevaluate if the proposed > > > > > approach shows negative impact on production traffic and I think > > > > > Cloudflare folks can give us the results soon. > > > > Do you prefer we also switch to using a mutex (with preemption > > > > disabled) to avoid the scenario Michal described where flushers give > > > > up the lock and sleep resulting in an unbounded wait time in the worst > > > > case? > > > Locking with mutex with preemption disabled is an oxymoron. > > I believe Yosry wanted to disable preemption _after_ the lock is taken > > to reduce the time spent while it is held. The idea to use the mutex is > > to reduce spinning and more importantly to get rid of lock dropping > > part. It is not really clear (but unlikely) we can drop it while > > preserving the spinlock as the thing scales with O(#cgroups x #cpus) > > in the worst case. > > As I have said later in my email, I am not against disabling preemption > selectively on some parts of the lock critical section where preemption is > undesirable. However, I am against disabling preemption for the whole > duration of the code where the mutex lock is held as it defeats the purpose > of using mutex in the first place. I certainly agree this is an antipattern. -- Michal Hocko SUSE Labs