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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id C6559C433F5 for ; Tue, 8 Feb 2022 16:32:42 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0816D6B0078; Tue, 8 Feb 2022 11:32:42 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 0075E6B007B; Tue, 8 Feb 2022 11:32:41 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E11BC6B007D; Tue, 8 Feb 2022 11:32:41 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.25]) by kanga.kvack.org (Postfix) with ESMTP id CBC886B0078 for ; Tue, 8 Feb 2022 11:32:41 -0500 (EST) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 9343755B for ; Tue, 8 Feb 2022 16:32:41 +0000 (UTC) X-FDA: 79120156122.02.B0F004C Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf02.hostedemail.com (Postfix) with ESMTP id 0A78880015 for ; Tue, 8 Feb 2022 16:32:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644337960; 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=BtkJvAo+N9u8Ul05NhvoL9QDcGnWfbGW9BYYZdFX1lY=; b=IbdOnHsp8rHImQLEuhPnpb1Jl5WRprTEMYOUrokXzG1CxbuDbVY/eAVYw/fe2MzI4Ag7N/ RgY/xvguoH/rqU2Wh5pm6TtMUxYXOFbnPftT6rdSQ6MeDQ8MDk4Sq6QrMwDnx/4HqI55vH 6ykdwcOiyARWt4MjODgtuCshXDQ+y1g= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-325-Yz1WSJKINhyj0X1oRqdZwg-1; Tue, 08 Feb 2022 11:32:37 -0500 X-MC-Unique: Yz1WSJKINhyj0X1oRqdZwg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 189D48519E0; Tue, 8 Feb 2022 16:32:35 +0000 (UTC) Received: from fuller.cnet (ovpn-112-4.gru2.redhat.com [10.97.112.4]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6E8B42A174; Tue, 8 Feb 2022 16:32:34 +0000 (UTC) Received: by fuller.cnet (Postfix, from userid 1000) id 5BB70416D5DD; Tue, 8 Feb 2022 12:47:10 -0300 (-03) Date: Tue, 8 Feb 2022 12:47:10 -0300 From: Marcelo Tosatti To: Nicolas Saenz Julienne Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, frederic@kernel.org, tglx@linutronix.de, mgorman@suse.de, linux-rt-users@vger.kernel.org, vbabka@suse.cz, cl@linux.com, paulmck@kernel.org, willy@infradead.org Subject: Re: [PATCH 2/2] mm/page_alloc: Add remote draining support to per-cpu lists Message-ID: References: <20220208100750.1189808-1-nsaenzju@redhat.com> <20220208100750.1189808-3-nsaenzju@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220208100750.1189808-3-nsaenzju@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Rspamd-Queue-Id: 0A78880015 X-Rspam-User: Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=IbdOnHsp; spf=none (imf02.hostedemail.com: domain of mtosatti@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: fc66ym69j5bt3mphb8f78bi4ao3yop7c X-Rspamd-Server: rspam04 X-HE-Tag: 1644337960-440475 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Feb 08, 2022 at 11:07:50AM +0100, Nicolas Saenz Julienne wrote: > The page allocator's per-cpu page lists (pcplists) are currently > protected using local_locks. While performance savvy, this doesn't allow > for remote access to these structures. CPUs requiring system-wide > changes to the per-cpu lists get around this by scheduling > workers on each CPU. That said, some setups like NOHZ_FULL CPUs, > aren't well suited to this since they can't handle interruptions > of any sort. > > To mitigate this, replace the current draining mechanism with one that > allows remotely draining the lists: > > - Each CPU now has two pcplists pointers: one that points to a pcplists > instance that is in-use, 'pcp->lp', another that points to an idle > and empty instance, 'pcp->drain'. CPUs access their local pcplists > through 'pcp->lp' and the pointer is dereferenced atomically. > > - When a CPU decides it needs to empty some remote pcplists, it'll > atomically exchange the remote CPU's 'pcp->lp' and 'pcp->drain' > pointers. A remote CPU racing with this will either have: > > - An old 'pcp->lp' reference, it'll soon be emptied by the drain > process, we just have to wait for it to finish using it. > > - The new 'pcp->lp' reference, that is, an empty pcplists instance. > rcu_replace_pointer()'s release semantics ensures any prior > changes will be visible by the remote CPU, for example: changes > to 'pcp->high' and 'pcp->batch' when disabling the pcplists. > > - The CPU that started the drain can now wait for an RCU grace period > to make sure the remote CPU is done using the old pcplists. > synchronize_rcu() counts as a full memory barrier, so any changes the > local CPU makes to the soon to be drained pcplists will be visible to > the draining CPU once it returns. > > - Then the CPU can safely free the old pcplists. Nobody else holds a > reference to it. Note that concurrent access to the remote pcplists > drain is protected by the 'pcpu_drain_mutex'. > > >From an RCU perspective, we're only protecting access to the pcplists > pointer, the drain operation is the writer and the local_lock critical > sections are the readers. RCU guarantees atomicity both while > dereferencing the pcplists pointer and replacing it. It also checks for > RCU critical section/locking correctness, as all readers have to hold > their per-cpu pagesets local_lock, which also counts as a critical > section from RCU's perspective. > > >From a performance perspective, on production configurations, the patch > adds an extra dereference to all hot paths (under such circumstances > rcu_dereference() will simplify to READ_ONCE()). Extensive measurements > have been performed on different architectures to ascertain the > performance impact is minimal. Most platforms don't see any difference > and the worst-case scenario shows a 1-3% degradation on a page > allocation micro-benchmark. See cover letter for in-depth results. > > Accesses to the pcplists like the ones in mm/vmstat.c don't require RCU > supervision since they can handle outdated data, but they do use > rcu_access_pointer() to avoid compiler weirdness make sparse happy. > > Note that special care has been taken to verify there are no races with > the memory hotplug code paths. Notably with calls to zone_pcp_reset(). > As Mel Gorman explains in a previous patch[1]: "The existing hotplug > paths guarantees the pcplists cannot be used after zone_pcp_enable() > [the one in offline_pages()]. That should be the case already because > all the pages have been freed and there is no page to put on the PCP > lists." > > All in all, this technique allows for remote draining on all setups with > an acceptable performance impact. It benefits all sorts of use cases: > low-latency, real-time, HPC, idle systems, KVM guests. > > [1] 8ca559132a2d ("mm/memory_hotplug: remove broken locking of zone PCP > structures during hot remove") > > Signed-off-by: Nicolas Saenz Julienne > --- > > Changes since RFC: > - Avoid unnecessary spin_lock_irqsave/restore() in free_pcppages_bulk() > - Add more detail to commit and code comments. > - Use synchronize_rcu() instead of synchronize_rcu_expedited(), the RCU > documentation says to avoid it unless really justified. I don't think > it's justified here, if we can schedule and join works, waiting for > an RCU grace period is OK. https://patchwork.ozlabs.org/project/netdev/patch/1306228052.3026.16.camel@edumazet-laptop/ Adding 100ms to direct reclaim path might be problematic. It will also slowdown kcompactd (note it'll call drain_all_pages for each zone). > - Avoid sparse warnings by using rcu_access_pointer() and > rcu_dereference_protected(). > > include/linux/mmzone.h | 22 +++++- > mm/page_alloc.c | 155 ++++++++++++++++++++++++++--------------- > mm/vmstat.c | 6 +- > 3 files changed, 120 insertions(+), 63 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index b4cb85d9c6e8..b0b593fd8e48 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -388,13 +388,31 @@ struct per_cpu_pages { > short expire; /* When 0, remote pagesets are drained */ > #endif > > - struct pcplists *lp; > + /* > + * As a rule of thumb, any access to struct per_cpu_pages's 'lp' has > + * happen with the pagesets local_lock held and using > + * rcu_dereference_check(). If there is a need to modify both > + * 'lp->count' and 'lp->lists' in the same critical section 'pcp->lp' > + * can only be derefrenced once. See for example: Typo.