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imf24.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf24.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1716183226; a=rsa-sha256; cv=none; b=8LoBALiw0JilAGjVLE+kBrFbVFjH4IhBadEYlUzXZN4qWJxycpM0Laqfujyh6yOfAj3ueL hZ3RO3VTxyfjvIl0Xq2puXDPQ9usA/tA2tlakDUBnC2pPSh8egkq6caPgoJcrInPg1tG/f b6rG2owLpdi7Q2+hnbGIWgAgCZe3VVE= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 02867FEC; Sun, 19 May 2024 22:34:08 -0700 (PDT) Received: from [10.162.40.20] (e116581.arm.com [10.162.40.20]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 52C203F641; Sun, 19 May 2024 22:33:41 -0700 (PDT) Message-ID: <4bef9468-a403-4bb5-940d-aacb611f28d1@arm.com> Date: Mon, 20 May 2024 11:03:38 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] selftests/mm: compaction_test: Fix trivial test success and reduce probability of OOM-killer invocation To: Andrew Morton Cc: shuah@kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Anshuman.Khandual@arm.com, sjayaram@akamai.com, stable@vger.kernel.org References: <20240515093633.54814-1-dev.jain@arm.com> <20240515093633.54814-3-dev.jain@arm.com> <20240519170357.757d30aac192c686f10d709c@linux-foundation.org> Content-Language: en-US From: Dev Jain In-Reply-To: <20240519170357.757d30aac192c686f10d709c@linux-foundation.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 9013E180009 X-Stat-Signature: w8zqj83auaos5nqkcnwp73mopbywbmez X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1716183226-119588 X-HE-Meta: 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 vTBEWJbL S5BmfzlcV2CvRwlB75tQeMw6f9Jw5BL2l0lFSfAdcYBneug8VBa/3zg7QuihOwy6hlDmwU35R3bz5BrJvReN+61SprSs9nK8cvMk38M3lNSRg+dyBiVfPXIxqNhd+ZDRjqv0wRuXMKUf4hUDXwSOTowdbg3aB+AgfhuF/zNMPNtsP4VA2ae/pOSu3DcuoB67wQGeGJMFyNgieZnD7eXqCIzPkhGdSahdA8C5Z05+KJPwZg7fROelVdrM58D+UGmxsCdzk 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 5/20/24 05:33, Andrew Morton wrote: > On Wed, 15 May 2024 15:06:33 +0530 Dev Jain wrote: > >> Reset nr_hugepages to zero before the start of the test. >> >> If a non-zero number of hugepages is already set before the start of the >> test, the following problems arise: >> >> - The probability of the test getting OOM-killed increases. >> Proof: The test wants to run on 80% of available memory to prevent >> OOM-killing (see original code comments). Let the value of mem_free at the >> start of the test, when nr_hugepages = 0, be x. In the other case, when >> nr_hugepages > 0, let the memory consumed by hugepages be y. In the former >> case, the test operates on 0.8 * x of memory. In the latter, the test >> operates on 0.8 * (x - y) of memory, with y already filled, hence, memory >> consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 * x. Q.E.D >> >> - The probability of a bogus test success increases. >> Proof: Let the memory consumed by hugepages be greater than 25% of x, >> with x and y defined as above. The definition of compaction_index is >> c_index = (x - y)/z where z is the memory consumed by hugepages after >> trying to increase them again. In check_compaction(), we set the number >> of hugepages to zero, and then increase them back; the probability that >> they will be set back to consume at least y amount of memory again is >> very high (since there is not much delay between the two attempts of >> changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption). >> Therefore, >> c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3 >> hence, c_index can always be forced to be less than 3, thereby the test >> succeeding always. Q.E.D >> >> NOTE: This patch depends on the previous one. >> >> -int check_compaction(unsigned long mem_free, unsigned int hugepage_size) >> +int check_compaction(unsigned long mem_free, unsigned int hugepage_size, >> + int initial_nr_hugepages) >> { >> int fd, ret = -1; >> int compaction_index = 0; >> - char initial_nr_hugepages[10] = {0}; >> char nr_hugepages[10] = {0}; >> + char init_nr_hugepages[10] = {0}; >> + >> + sprintf(init_nr_hugepages, "%d", initial_nr_hugepages); > Well, [10] isn't really large enough. "-1111111111" requires 12 chars, > with the trailing \0. And I'd suggest an unsigned type and a %u - > negative initial_nr_hugepages doesn't make a lot of sense. > >> >> +int set_zero_hugepages(int *initial_nr_hugepages) >> +{ >> + int fd, ret = -1; >> + char nr_hugepages[10] = {0}; > Ditto? Sure, makes sense. I'll just change that to 20 and make it unsigned. > >> + fd = open("/proc/sys/vm/nr_hugepages", O_RDWR | O_NONBLOCK); >> + if (fd < 0) { >> + ksft_print_msg("Failed to open /proc/sys/vm/nr_hugepages: %s\n", >> + strerror(errno)); >> + goto out; >> + } >> + >> + if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) { >> + ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n", >> + strerror(errno)); >> + goto close_fd; >> + } >> + >> + lseek(fd, 0, SEEK_SET); >> + >> + /* Start with the initial condition of 0 huge pages */ >> + if (write(fd, "0", sizeof(char)) != sizeof(char)) { >> + ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n", >> + strerror(errno)); >> + goto close_fd; >> + } >> + >> + *initial_nr_hugepages = atoi(nr_hugepages); >> + ret = 0; >> + >> + close_fd: >> + close(fd); >> + >> + out: >> + return ret; >> +}