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a="193678982" X-IronPort-AV: E=Sophos;i="5.82,207,1613462400"; d="scan'208";a="193678982" Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Apr 2021 14:51:21 -0700 IronPort-SDR: z02cTHaY3bIgDxHaabqoQzzIiuAvHzAwZAaOdOHiRjasjBanJFuXdyrw6OBsuhXyCKTaN3/Zu6 IoU76fiwwpCA== X-IronPort-AV: E=Sophos;i="5.82,207,1613462400"; d="scan'208";a="441911540" Received: from mongkaiw-mobl.amr.corp.intel.com (HELO [10.209.62.147]) ([10.209.62.147]) by fmsmga004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Apr 2021 14:51:21 -0700 Subject: Re: [PATCH 02/10] mm/numa: automatically generate node migration order To: Oscar Salvador , Dave Hansen Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, shy828301@gmail.com, weixugc@google.com, rientjes@google.com, ying.huang@intel.com, dan.j.williams@intel.com, david@redhat.com References: <20210401183216.443C4443@viggo.jf.intel.com> <20210401183219.DC1928FA@viggo.jf.intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Stat-Signature: 9wbwyyoj6xnskcpyq67kyircbaiyjqny X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 40DA1C0007C0 Received-SPF: none (intel.com>: No applicable sender policy available) receiver=imf03; identity=mailfrom; envelope-from=""; helo=mga03.intel.com; client-ip=134.134.136.65 X-HE-DKIM-Result: none/none X-HE-Tag: 1617918681-940367 Content-Transfer-Encoding: quoted-printable 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 4/8/21 1:26 AM, Oscar Salvador wrote: > On Thu, Apr 01, 2021 at 11:32:19AM -0700, Dave Hansen wrote: >> The protocol for node_demotion[] access and writing is not >> standard. It has no specific locking and is intended to be read >> locklessly. Readers must take care to avoid observing changes >> that appear incoherent. This was done so that node_demotion[] >=20 > It might be just me being dense here, but that reads odd. >=20 > "Readers must take care to avoid observing changes that appear > incoherent" - I am not sure what is that supposed to mean. >=20 > I guess you mean readers of next_demotion_node()? > And if so, how do they have to take care? And what would apply for > "incoherent" terminology here? I've fleshed out the description a bit. I hope this helps? > Readers of node_demotion[] (such as next_demotion_node() callers) > must take care to avoid observing changes that appear incoherent. > For instance, even though no demotion cycles are allowed, it's > possible that a cycle could be observed. >=20 > Let's say that there are three nodes, A, B and C. node_demotion[] > is set up to have this path: > =20 > A -> B -> C >=20 > Suppose it was modified to instead represent this path: >=20 > A -> C -> B >=20 > There is nothing to stop a reader from seeing B->C and then a > moment later seeting C->B. That *appears* to be a cycle. This > can be avoided with RCU and will be implemented in a later patch. ... >> +again: >> + this_pass =3D next_pass; >> + next_pass =3D NODE_MASK_NONE; >> + /* >> + * To avoid cycles in the migration "graph", ensure >> + * that migration sources are not future targets by >> + * setting them in 'used_targets'. Do this only >> + * once per pass so that multiple source nodes can >> + * share a target node. >> + * >> + * 'used_targets' will become unavailable in future >> + * passes. This limits some opportunities for >> + * multiple source nodes to share a destination. >> + */ >> + nodes_or(used_targets, used_targets, this_pass); >> + for_each_node_mask(node, this_pass) { >> + int target_node =3D establish_migrate_target(node, &used_targets); >> + >> + if (target_node =3D=3D NUMA_NO_NODE) >> + continue; >> + >> + /* Visit targets from this pass in the next pass: */ >> + node_set(target_node, next_pass); >> + } >> + /* Is another pass necessary? */ >> + if (!nodes_empty(next_pass)) >=20 > When I read this I was about puzzled and it took me a while to figure > out how the passes were made. > I think this could benefit from a better explanation on how the passes > are being performed e.g: why next_pass should be empty before leaving. >=20 > Other than that looks good to me. I've tried to flesh out those comments to elaborate on what is going on: > /* > * Visit targets from this pass in the next pass. > * Eventually, every node will have been part of > * a pass, and will become set in 'used_targets'. > */ > node_set(target_node, next_pass); > } > /* > * 'next_pass' contains nodes which became migration > * targets in this pass. Make additional passes until > * no more migrations targets are available. > */ > if (!nodes_empty(next_pass)) > goto again; > }