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([2601:408:c101:1d00:6621:a07c:fed4:cbba]) by smtp.gmail.com with ESMTPSA id e9e14a558f8ab-3d7dc5826f5sm27429815ab.49.2025.04.14.06.15.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Apr 2025 06:15:59 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <6572da04-d6d6-4f5e-9f17-b22d5a94b9fa@redhat.com> Date: Mon, 14 Apr 2025 09:15:57 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs() To: =?UTF-8?Q?Michal_Koutn=C3=BD?= Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Tejun Heo , Shuah Khan , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <20250414021249.3232315-1-longman@redhat.com> <20250414021249.3232315-2-longman@redhat.com> In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: HT9Zy_E2UgabCSk5UEQSiNJd4CUQahSdQRBYpOpvEMI_1744636561 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 50A9B160011 X-Stat-Signature: wbkycx3ihetomnag6u6w16hdwwndpda5 X-HE-Tag: 1744636565-594356 X-HE-Meta: U2FsdGVkX1+qVVXxPzUtHqk9A4sMgSITiSY6DQ4V7tp4ozCOgStmKtyfLgyjOoOGVrhn6mjYSzlntn9G2DbfLije+FvKXLTtnaGjtmOugNcCN8QThCm5g/U9WszrzQV1N/TOPaOmhTEaSEjnHcLX0ewzvU+EIdn/HCXr3rZG+QiItNTC0URobtg3qIl+E152izWvOSVvc7AzbVj9vuSg7vf+1wJzG4qsJAGGkXTS4pBLtx/5FXeunRfIiMqTslXAPQTAwfq/ZCdMqCxam5oec4KRGVK9SIc1wkW+BZ0fh3qGoGjDnaLMyuQiYOTgaMp9gWAL7PdtxjUmNBy470N+rRthJzDgsIMtcrnqpj5o65Rc/Io6j8YfqR9MIxn+QpDbQV3v9Sv3Z/tbYLLXhkfaX2ICsxoaIJsK+zvdva19HJA7Z0hUzLGvAG133G4RNjRqrWv31d3DUHkP/LK8xBsBw8mw0kxR1hVIeFfTSNNDfEyEyKn4K6qTlEUkAKGKSBzjqkoYCWtO9jXzMB3Csl1PcjsomAC+HO5dZt56Fg36e2+AQmMoi4izXoP+/WHfdxaFppSB+jNao3iUXPpl0Qle2PFlTG30zHvBdNYLmlasYSi2SWcDaSeR8USLL/iWX965iAcrerwegYo3Itvp4A8QgJl1o5EOwXO+9r1y/nk7C0W/Q0txZ6zZFaysdqzXLc9Dfne2ayILbhDgXz3zfFqqS4BtcJJxumHr2MZWHkn3qhv5hIhH+i0xJ3BJ6yL8jW1gEM+GV+7dKaswAGVkyETVBwxoznSdHRkq81D14N1Jh1WDQCwk31Zo/6NsgZ6Hl+x50jzOyrcU1V8ALGLY6UGDTYVvwuqeaBKAlcPYPHtuNvbZ0ZX8fznr42cSSclj/fkbhfWktj0Mc7KYQMyAB+fCN4MfkE4uiMcC6vTsSXOixkiT8pNnU+Dpg3Q7Wj2Sg7qwqb1Vniy7t91ysjbQHVQ lmyf6T8p 4OTU5lFEfFau50hALksufRQlYjSVlv1ZeS/0NVanFBVXTxq6ZWxQwWPcyhhylcBqruFiJIVU8BVtME/sf2HvPwJqeHFjD/Jn/V9bKRNmga6RavKJfpakq2tYyP5HOr6eFaGiG 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: List-Subscribe: List-Unsubscribe: On 4/14/25 8:42 AM, Michal Koutný wrote: > On Sun, Apr 13, 2025 at 10:12:48PM -0400, Waiman Long wrote: >> 2) memory.low is set to a non-zero value but the cgroup has no task in >> it so that it has an effective low value of 0. Again it may have a >> non-zero low event count if memory reclaim happens. This is probably >> not a result expected by the users and it is really doubtful that >> users will check an empty cgroup with no task in it and expecting >> some non-zero event counts. > I think you want to distinguish "no tasks" vs "no usage" in this > paragraph. Good point. Will update it if I need to send a new version. >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -5963,6 +5963,10 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc) >> >> mem_cgroup_calculate_protection(target_memcg, memcg); >> >> + /* Skip memcg with no usage */ >> + if (!mem_cgroup_usage(memcg, false)) >> + continue; >> + >> if (mem_cgroup_below_min(target_memcg, memcg)) { > As I think more about this -- the idea expressed by the diff makes > sense. But is it really a change? > For non-root memcgs, they'll be skipped because 0 >= 0 (in > mem_cgroup_below_min()) and root memcg would hardly be skipped. I did see some low event in the no usage case because of the ">=" comparison used in mem_cgroup_below_min(). I originally planning to guard against the elow == 0 case but Johannes advised against it. > > >> --- a/tools/testing/selftests/cgroup/test_memcontrol.c >> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c >> @@ -380,10 +380,10 @@ static bool reclaim_until(const char *memcg, long goal); >> * >> * Then it checks actual memory usages and expects that: >> * A/B memory.current ~= 50M >> - * A/B/C memory.current ~= 29M >> - * A/B/D memory.current ~= 21M >> - * A/B/E memory.current ~= 0 >> - * A/B/F memory.current = 0 >> + * A/B/C memory.current ~= 29M [memory.events:low > 0] >> + * A/B/D memory.current ~= 21M [memory.events:low > 0] >> + * A/B/E memory.current ~= 0 [memory.events:low == 0 if !memory_recursiveprot, > 0 otherwise] > Please note the subtlety in my suggestion -- I want the test with > memory_recursiveprot _not_ to check events count at all. Because: > a) it forces single interpretation of low events wrt effective > low limit > b) effective low limit should still be 0 in E in this testcase > (there should be no unclaimed protection of C and D). Yes, low event count for E is 0 in the !memory_recursiveprot case, but C/D still have low events and setting no_low_events_index to -1 will fail the test and it is not the same as not checking low event counts at all. Cheers, Longman