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 DF7AAC433F5 for ; Mon, 16 May 2022 10:53:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242857AbiEPKx2 (ORCPT ); Mon, 16 May 2022 06:53:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42134 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242864AbiEPKxW (ORCPT ); Mon, 16 May 2022 06:53:22 -0400 Received: from outbound-smtp60.blacknight.com (outbound-smtp60.blacknight.com [46.22.136.244]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9DE0C25EB9 for ; Mon, 16 May 2022 03:53:14 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail02.blacknight.ie [81.17.254.11]) by outbound-smtp60.blacknight.com (Postfix) with ESMTPS id DEFA5FA878 for ; Mon, 16 May 2022 11:53:12 +0100 (IST) Received: (qmail 8810 invoked from network); 16 May 2022 10:53:12 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.198.246]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 16 May 2022 10:53:12 -0000 Date: Mon, 16 May 2022 11:53:11 +0100 From: Mel Gorman To: Andrew Morton Cc: Nicolas Saenz Julienne , Marcelo Tosatti , Vlastimil Babka , Michal Hocko , LKML , Linux-MM Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3 Message-ID: <20220516105311.GL3441@techsingularity.net> References: <20220512085043.5234-1-mgorman@techsingularity.net> <20220512124325.751781bb88ceef5c37ca653e@linux-foundation.org> <20220513142330.GI3441@techsingularity.net> <20220513123805.41e560392d028c271b36847d@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20220513123805.41e560392d028c271b36847d@linux-foundation.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 13, 2022 at 12:38:05PM -0700, Andrew Morton wrote: > > The sentence can be dropped because it adds little and is potentially > > confusing. The PCP being safe to access remotely is specific to the > > context of the CPU being hot-removed and there are other special corner > > cases like zone_pcp_disable that modifies a per-cpu structure remotely > > but not in a way that causes corruption. > > OK. I pasted in your para from the other email. Current 0/n blurb: > > Some setups, notably NOHZ_FULL CPUs, may be running realtime or > latency-sensitive applications that cannot tolerate interference due to > per-cpu drain work queued by __drain_all_pages(). Introduce a new > mechanism to remotely drain the per-cpu lists. It is made possible by > remotely locking 'struct per_cpu_pages' new per-cpu spinlocks. This has > two advantages, the time to drain is more predictable and other unrelated > tasks are not interrupted. > > This series has the same intent as Nicolas' series "mm/page_alloc: Remote > per-cpu lists drain support" -- avoid interference of a high priority task > due to a workqueue item draining per-cpu page lists. While many workloads > can tolerate a brief interruption, it may cause a real-time task running > on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining is > non-deterministic. > > Currently an IRQ-safe local_lock protects the page allocator per-cpu > lists. The local_lock on its own prevents migration and the IRQ disabling > protects from corruption due to an interrupt arriving while a page > allocation is in progress. > > This series adjusts the locking. A spinlock is added to struct > per_cpu_pages to protect the list contents while local_lock_irq continues > to prevent migration and IRQ reentry. This allows a remote CPU to safely > drain a remote per-cpu list. > > This series is a partial series. Follow-on work should allow the > local_irq_save to be converted to a local_irq to avoid IRQs being > disabled/enabled in most cases. Consequently, there are some TODO > comments highlighting the places that would change if local_irq was used. > However, there are enough corner cases that it deserves a series on its > own separated by one kernel release and the priority right now is to avoid > interference of high priority tasks. > Looks good, thanks! -- Mel Gorman SUSE Labs