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 9DCDEC433FE for ; Wed, 16 Nov 2022 14:57:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233576AbiKPO5V (ORCPT ); Wed, 16 Nov 2022 09:57:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48704 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233632AbiKPO5R (ORCPT ); Wed, 16 Nov 2022 09:57:17 -0500 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 138DCFADF; Wed, 16 Nov 2022 06:57:16 -0800 (PST) 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-out2.suse.de (Postfix) with ESMTPS id 98D361F94F; Wed, 16 Nov 2022 14:57:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1668610635; 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=3vl4NdpO4irC1eRJoV821C0JFp7+4A15d5hSm1FQvTo=; b=eqaB6EP2GKiw5QP6JiW0cWOM1NG4anZRyh1calWKdEwUX95RbEKck+SxlKqWruwGYlnjRW InAsn8C+8XCSZZ23Mcg3b7PsV6E7utY9JOJX4lXDMQlPNUCOIwlxjZtQp2QRMW+DkxU1bQ lzj7mbU+Msd0KWWV5/hZ7unfoL4q6B4= 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 7A24113480; Wed, 16 Nov 2022 14:57:15 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id qwJoG0v6dGOYYgAAMHmgww (envelope-from ); Wed, 16 Nov 2022 14:57:15 +0000 Date: Wed, 16 Nov 2022 15:57:14 +0100 From: Michal Hocko To: Zhongkun He Cc: Andrew Morton , corbet@lwn.net, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [External] Re: [PATCH v2] mm: add new syscall pidfd_set_mempolicy(). Message-ID: References: <20221111084051.2121029-1-hezhongkun.hzk@bytedance.com> <20221111112732.30e1696bcd0d5b711c188a9a@linux-foundation.org> <3a3b4f5b-14d1-27d8-7727-cf23da90988f@bytedance.com> <82c9c89c-aee2-08a3-e562-359631bb0137@bytedance.com> <0bd0b744-3d97-b4c3-a4fb-6040f8f8024a@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0bd0b744-3d97-b4c3-a4fb-6040f8f8024a@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Wed 16-11-22 19:28:10, Zhongkun He wrote: > Hi Michal, I've done the performance testing, please check it out. > > > > Yes this is all understood but the level of the overhead is not really > > > clear. So the question is whether this will induce a visible overhead. > > > Because from the maintainability point of view it is much less costly to > > > have a clear life time model. Right now we have a mix of reference > > > counting and per-task requirements which is rather subtle and easy to > > > get wrong. In an ideal world we would have get_vma_policy always > > > returning a reference counted policy or NULL. If we really need to > > > optimize for cache line bouncing we can go with per cpu reference > > > counters (something that was not available at the time the mempolicy > > > code has been introduced). > > > > > > So I am not saying that the task_work based solution is not possible I > > > just think that this looks like a good opportunity to get from the > > > existing subtle model. > > Test tools: > numactl -m 0-3 ./run-mmtests.sh -n -c configs/config-workload- > aim9-pagealloc test_name > > Modification: > Get_vma_policy(), get_task_policy() always returning a reference > counted policy, except for the static policy(default_policy and > preferred_node_policy[nid]). It would be better to add the patch that has been tested. > All vma manipulation is protected by a down_read, so mpol_get() > can be called directly to take a refcount on the mpol. but there > is no lock in task->mempolicy context. > so task->mempolicy should be protected by task_lock. > > struct mempolicy *get_task_policy(struct task_struct *p) > { > struct mempolicy *pol; > int node; > > if (p->mempolicy) { > task_lock(p); > pol = p->mempolicy; > mpol_get(pol); > task_unlock(p); > if (pol) > return pol; > } One way to deal with that would be to use a similar model as css_tryget Btw. have you tried to profile those slowdowns to identify hotspots? Thanks -- Michal Hocko SUSE Labs