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Fri, 12 Jul 2024 07:19:11 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 8oyjG+/YkGZHbwAAD6G6ig (envelope-from ); Fri, 12 Jul 2024 07:19:11 +0000 Message-ID: Date: Fri, 12 Jul 2024 09:19:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] revert unconditional slab and page allocator fault injection calls Content-Language: en-US To: Andrew Morton Cc: Mateusz Guzik , Akinobu Mita , Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Christoph Lameter , David Rientjes , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-mm@kvack.org References: <20240711-b4-fault-injection-reverts-v1-0-9e2651945d68@suse.cz> <20240711123637.508e81d2b25ac8e0c9b726c0@linux-foundation.org> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: bzspskrp4p8twapabuukjf7cyrmgr5ce X-Rspamd-Queue-Id: 9A7114000C X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1720768753-463999 X-HE-Meta: U2FsdGVkX18PXAyPDS6re8hxpFAGnH4B/glyE8xuc1GQBOv2nlkd+OnDEexshE/69Pzj6cWm2aIqHX/yJdg3VyQ2e50DYN93zmAhzW18WKjSfoFWL2ZcXvw++HhhLoVeUfdfmF9DYorLige7shU9qkxzJxfOB5UscDwbb5R7YM/RQ0I9uE9zQS/GXdHJckwjsg+OxeZb3JOp3NtVVPfWRrmjXdSEN6VzOpoGDzvUX8NC5cGtT893WTixawn9JzQxfe1gLp0h3UQ5/M0lMO+sDAVWaNHymkuiK5FAHNOpu0zU8kVuDFB0R5ClUKu2qOMQvn4xTNIHmv3J86C7CV5vY/dfZB3D/EK6bVikviQ34nZK/2nthL2JtXqYnYls2bZsOq1doZCi++IMpmAq+DBALOWo5D6S59BtYikCcNiFJqoBjN8rSgNhh4xRFTlRD/bqBUuYpKc2jkpuYfr+1QaO/bC9QRHhr6wiCiqynIdQB3u+UawJszpu5kB5mqMZVfQsmzJMJRzsKPAKJsMExnmQHcLGRQ8YypiGq/X/9CSOrRp4yvV/TyCDXnonBAMm0HLlD5YghL6m2b2SsFTwPEvLWKsnlLsdHFrE7xAcmlXJv2yXPg/CUDBzAb5Z80215Rt9tFbW+wG5QcdE/tGQKPO611UZt/PdMw2JY9wBiVuoHeeKnazVSzOoMoVg3O7mtdo9iLhm9nvk44hWq4vYJ6d2okw5CWuPSXMGoBqdQp9hOP75i3qAg1vg//WM+MFUKZ8+i0S5npv2Pv3gTuog8cvgGXtNQXE2223uE4bRS5kBxwj3Eo5jLYeQefokeZ5IsJKQLLYJKGEZ5vIRgOgVXspltb6z7ODfj1+8VsBZ2RKx7I7KZGCpRypoPbIcQ7vlPMYmXkBxYIesM+9/yRGnsv7oqX3wAniAwPbOrULwBoYCfP2xQV/4oCJ2nZVNvBdxbuB5onjK2DMIq+OwPczbVR7 xNbg66H1 oBruQqcfdzETH8oXe7ZRXIkEyojjtqQipQiXrpRLGWQ+jiiFJUYJO+lcKe86wcsRcg8MljBB3tCtBbjlCFhZxVHQzushsfzdSw6qRSfbeHNoWfvZDEAseKdJi/iWB6jTN45K9N0teMtQkMyF9TS0OH9aMZguCL7n+a3UaIY3bkhxHg7nLJ807QN3cc1M/BtGxHdnErdWzZYcLtMqAQDh08fJ0d+u6gm6Yvl42cEJht+B6UQGxALBz9Mid/+2IQo5OhCPY9HBBTOVQ62HCwDb3FeB5V0xoiDqB+8dc7i1sEyTSad0G38RGplgat0x5ys/N+rf7f7lmO8iX6y14hX6THf2sHk71EZs2wfBFyTHlU9/PBgclTGiUtaXFfTrmlQ+0AVowAivoCLROykY= 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 7/11/24 9:36 PM, Andrew Morton wrote: > On Thu, 11 Jul 2024 18:35:29 +0200 Vlastimil Babka wrote: > >> These two patches largely revert commits that added function call >> overhead into slab and page allocation hotpaths and that cannot be >> currently disabled even though related CONFIG_ options do exist. > > Five years ago. I assume the overall overhead is small? Well, what made me look into this in the first place was seeing should_failslab() in perf profiles at 1-2% even though it was an empty function that just immediately returned. In [1] I posted some measurements that was not even a microbenchmark: To demonstrate the reduced overhead of calling an empty should_failslab() function, a kernel build with CONFIG_FUNCTION_ERROR_INJECTION enabled but CONFIG_FAILSLAB disabled, and CPU mitigations enabled, was used in a qemu-kvm (virtme-ng) on AMD Ryzen 7 2700 machine, and execution of a program trying to open() a non-existent file was measured 3 times: for (int i = 0; i < 10000000; i++) { open("non_existent", O_RDONLY); } After this patch, the measured real time was 4.3% smaller. Using perf profiling it was verified that should_failslab was gone from the profile. Later I found that this CPU mitigations were really important here as function calls are more expensive. With them disabled that benchmark was in a noise, so I wasn't sure about claiming that number in the patch itself. But I assume a microbenchmark would still demonstrate some overhead. Yet ultimately I think the overhead is just plain unnecessary to pay when error injection is not being performed, and also CPU mitigations enabled are usually the default, so it's best get rid of it. [1] https://lore.kernel.org/all/20240620-fault-injection-statickeys-v2-0-e23947d3d84b@suse.cz/#t