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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id 14-20020a05620a040e00b006fbb4b98a25sm9597056qkp.109.2023.03.01.14.57.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Mar 2023 14:57:09 -0800 (PST) Date: Wed, 1 Mar 2023 17:57:08 -0500 From: Peter Xu To: Marcelo Tosatti Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 09/11] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold Message-ID: References: <20230209150150.380060673@redhat.com> <20230209153204.873999366@redhat.com> MIME-Version: 1.0 In-Reply-To: <20230209153204.873999366@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 17C91180009 X-Stat-Signature: 3k5i1u4ka68h5re9xdj9wrew6ju7srek X-HE-Tag: 1677711432-914740 X-HE-Meta: U2FsdGVkX18+qh9We1o5wOP58wOdoLWNGfMZK1eXV1sc3Jb95QAIm9lCXUKMyZyn1iJddzgxDqaULZmi2feKId1P4jPXt1qxI/Tpp+ObETRjOuZ1tJ+14LM+H3AP3qyLpLACQXrqpSikt7Vs/LRsOfxP/1gI7bAECs3W64XjgBM3EF7dGWeSO6udO668pUj8IW7cNEvNOFNUYtCeSl+1mkP/SGhu/GLDZKd1nAxcf1ZPH8LRGPobJ60ITZw+ZKq9lNkNmLT90TayiG+s8/DtfBdRRLwC5dVOlTffEMJy9bzZ5/UxJo3JguGI9I+5IgpPSxpB16Z53XL2P+3rhOQ84M8IB6/SiU6mBMGv3VYxEEsOx44L44PPjvByI8VV2VrzPcJj3GbQwK03gpmYb/frefT/qSkQMUXerqScLEOOBtKSj0nfx9vIsgWoZ3iSldiFbiCe6CTgXbYlAe4UxBJnfeQ4VTRinzTWt2d+yR46FY3nLVkavYZCRQ+0IPXN8Yk22qL4umRf96vRrbNMT3/bl3VtPbDVajvaVw4+qPe/tVNJSzsnqGldDp2UM6XhRhiVhn5IKXqxSKYbfVP6Zj0IPj4bSmF7/3DF2j0z5WNTjsJ07zL8+Yt+E+/Ru60O3DwwBip9HfY9UoS2VS3qzB8aQPJm9Cwp4Md4IeD/1vvewo/Y89OG+29DRKfVNb5g9PJ8pa2CRGiBc6hAGXsGr5wN31Vn8Cj5V5Z3NBUvk5X54y6HUSMmxcOdeHmBjmakcceR5ZOxJlLGbnCfhUgMnpIj4d+ToNmbV4BniqEdeHjR6bpdvPnelVBzFaAWXLqHaRfT5K8J+DotH1moPzf5Abek5tSoZ96FwT+z4pyA9G4OvsbEYisS79VcF4QsyMNfemGU28CjxbJQcSifzb5HZF6wGcpkUX32aVePruZ74oIQQrVEUZPYAoIlJpnfEjgJbszEO6o1YDDj4SYXbNJtnRr UcrWc0J6 UlnX24j3s3GK9tpVq/hhP7b7/RX+0g+ymKFfQqE93Ufd/kDBx+bWsIn0pmQ4YtJbC/7FcXgYcBM+J2wmJzhIRdOfAuZPjJ+tOh4YS2RDsS7IyWAaa00nmRNpjCGi3vviGsVHKP9aiXOLf9GTordERk+zWYn2tCjchz/E+NPjQpMFklaaQ2NCF/7qN+UtZhEEp+AhRNGbfXbxHmZf3Ao9YgFbJpf10xmgDPs9KVr+1+fLdUkSAFqM6ENIb5H5a81ZtC6X8SUMW9QqzWKQR3LqlUvseEPhvLgskVZZ9S7EG7P19di55FXKIvoAUSGfem0lT6T/TmnCDb5M9CMybVmp8s/ogMBi6Mtes9LZ4x4d8pjlhfQP5fdaS4CynqGPb+sr1F0rg+zm/zlPeApus6G01+VCJKpzhmHJBFV4dNFtUZhd4Mhc= 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 Thu, Feb 09, 2023 at 12:01:59PM -0300, Marcelo Tosatti wrote: > /* > - * Fold the data for an offline cpu into the global array. > + * Fold the data for a cpu into the global array. > * There cannot be any access by the offline cpu and therefore > * synchronization is simplified. > */ > @@ -906,8 +906,9 @@ void cpu_vm_stats_fold(int cpu) > if (pzstats->vm_stat_diff[i]) { > int v; > > - v = pzstats->vm_stat_diff[i]; > - pzstats->vm_stat_diff[i] = 0; > + do { > + v = pzstats->vm_stat_diff[i]; > + } while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0)); IIUC try_cmpxchg will update "v" already, so I'd assume this'll work the same: while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0)); Then I figured, maybe it's easier to use xchg()? I've no knowledge at all on cpu offline code, so sorry if this will be a naive question. But from what I understand this should not be touched by anyone else. Reasons: (1) cpu_vm_stats_fold() is only called in page_alloc_cpu_dead(), and the comment says: /* * Zero the differential counters of the dead processor * so that the vm statistics are consistent. * * This is only okay since the processor is dead and cannot * race with what we are doing. */ cpu_vm_stats_fold(cpu); so.. I think that's what it says.. (2) If someone can modify the dead cpu's vm_stat_diff, what guarantees it won't be e.g. boosted again right after try_cmpxchg() / xchg() returns? What to do with the left-overs? Thanks, -- Peter Xu