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 2F335C5479D for ; Wed, 11 Jan 2023 17:10:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232616AbjAKRKF (ORCPT ); Wed, 11 Jan 2023 12:10:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39240 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238488AbjAKRJk (ORCPT ); Wed, 11 Jan 2023 12:09:40 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3A005B868 for ; Wed, 11 Jan 2023 09:08:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1673456902; h=from:from:reply-to:subject:subject: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=IducYfddzH6lMplxq5YrX0OgBDUYvrG6JVse4EUppbE=; b=Ayzxx8Dpw4lk2haLG4gg3SYoVuwF3hLWexFsYjLpwV3H+IUsWJkK41QQzkSOtQkzdtbHNz 21Q48iAGptpkO3704kt4iThT1uoctT173EDBBE6V4az8AdUKGYzjT2EsC22vi2rj9KC6OA IZDOLNtU5V/V9GwKme+TQNz46MN4Fvs= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-84-h6Akq_tMN7yf2EHiQ2PiFw-1; Wed, 11 Jan 2023 12:08:06 -0500 X-MC-Unique: h6Akq_tMN7yf2EHiQ2PiFw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5E6463C4220B; Wed, 11 Jan 2023 17:08:05 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 100EBC0328F; Wed, 11 Jan 2023 17:08:05 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 8E81E401A035B; Wed, 11 Jan 2023 14:07:47 -0300 (-03) Date: Wed, 11 Jan 2023 14:07:47 -0300 From: Marcelo Tosatti To: Christoph Lameter Cc: Frederic Weisbecker , atomlin@atomlin.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v13 2/6] mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies Message-ID: References: <20230105125218.031928326@redhat.com> <20230105125248.813825852@redhat.com> <7c2af941-42a9-a59b-6a20-b331a4934a3@gentwo.de> <60183179-3a28-6bf9-a6ab-8a8976f283d@gentwo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <60183179-3a28-6bf9-a6ab-8a8976f283d@gentwo.de> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 11, 2023 at 09:42:18AM +0100, Christoph Lameter wrote: > On Tue, 10 Jan 2023, Marcelo Tosatti wrote: > > > > The basic primitives add a lot of weight. > > > > Can't see any alternative given the necessity to avoid interruption > > by the work to sync per-CPU vmstats to global vmstats. > > this_cpu operations are designed to operate on a *single* value (a counter) and can > be run on an arbitrary cpu, There is no preemption or interrupt > disable required since the counters of all cpus will be added up at the > end. > > You want *two* values (the counter and the dirty flag) to be modified > together and want to use the counters/flag to identify the cpu where > these events occurred. this_cpu_xxx operations are not suitable for that > purpose. You would need a way to ensure that both operations occur on the > same cpu. Which is either preempt_disable (CONFIG_HAVE_CMPXCHG_LOCAL case), or local_irq_disable (!CONFIG_HAVE_CMPXCHG_LOCAL case). > > > > And the pre cpu atomic updates operations require the modification > > > of multiple values. The operation > > > cannot be "atomic" in that sense anymore and we need some other form of > > > synchronization that can > > > span multiple instructions. > > > > So use this_cpu_cmpxchg() to avoid the overhead. Since we can no longer > > count on preremption being disabled we still have some minor issues. > > The fetching of the counter thresholds is racy. > > A threshold from another cpu may be applied if we happen to be > > rescheduled on another cpu. However, the following vmstat operation > > will then bring the counter again under the threshold limit. > > > > Those small issues are gone, OTOH. > > Well you could use this_cpu_cmpxchg128 to update a 64 bit counter and a > flag at the same time. But then you transform the "per-CPU vmstat is dirty" bit (bool) into a number of flags that must be scanned (when returning to userspace). Which increases the overhead of a fast path (return to userspace). > Otherwise you will have to switch off preemption or > interrupts when incrementing the counters and updating the dirty flag. > > Thus you do not really need the this_cpu operations anymore. It would > best to use a preempt_disable section and uuse C operators -- ++ for the > counter and do regular assignment for the flag. OK, can replace this_cpu operations with this_cpu_ptr + standard C operators (and in fact can do that for interrupt disabled functions as well, that is CONFIG_HAVE_CMPXCHG_LOCAL not defined). Is that it?