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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4356992201csm6906850f8f.2.2026.01.16.10.37.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Jan 2026 10:37:46 -0800 (PST) Date: Fri, 16 Jan 2026 18:37:44 +0000 From: David Laight To: Holger Dengler Cc: Eric Biggers , Ard Biesheuvel , "Jason A . Donenfeld" , Herbert Xu , Harald Freudenberger , linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org Subject: Re: [PATCH v1 1/1] lib/crypto: tests: Add KUnit tests for AES Message-ID: <20260116183744.04781509@pumpkin> In-Reply-To: <389595e9-e13a-42e3-b0ff-9ca0dd3effe3@linux.ibm.com> References: <20260115183831.72010-1-dengler@linux.ibm.com> <20260115183831.72010-2-dengler@linux.ibm.com> <20260115204332.GA3138@quark> <20260115220558.25390c0e@pumpkin> <389595e9-e13a-42e3-b0ff-9ca0dd3effe3@linux.ibm.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 16 Jan 2026 18:31:58 +0100 Holger Dengler wrote: > Hi David, > > On 15/01/2026 23:05, David Laight wrote: > > On Thu, 15 Jan 2026 12:43:32 -0800 > > Eric Biggers wrote: > >>> +static void benchmark_aes(struct kunit *test, const struct aes_testvector *tv) > >>> +{ > >>> + const size_t num_iters = 10000000; > >> > >> 10000000 iterations is too many. That's 160 MB of data in each > >> direction per AES key length. Some CPUs without AES instructions can do > >> only ~20 MB AES per second. In that case, this benchmark would take 16 > >> seconds to run per AES key length, for 48 seconds total. > > > > Probably best to first do a test that would take a 'reasonable' time > > on a cpu without AES. If that is 'very fast' then do a longer test > > to get more accuracy on a faster implementation. > > > >> > >> hash-test-template.h and crc_kunit.c use 10000000 / (len + 128) > >> iterations. That would be 69444 in this case (considering len=16), > >> which is less than 1% of the iterations you've used. Choosing a number > >> similar to that would seem more appropriate. > >> > >> Ultimately these are just made-up numbers. But I think we should aim > >> for the benchmark test in each KUnit test suite to take less than a > >> second or so. The existing tests roughly achieve that, whereas it seems > >> this one can go over it by quite a bit due to the 10000000 iterations. > > > > Even 1 second is a long time, you end up getting multiple interrupts included. > > I think a lot of these benchmarks are far too long. > > Timing differences less that 1% can be created by scheduling noise. > > Running a test that takes 200 'quanta' of the timer used has an > > error margin of under 1% (100 quanta might be enough). > > While the kernel timestamps have a resolution of 1ns the accuracy is worse. > > If you run a test for even just 10us you ought to get reasonable accuracy > > with a reasonable hope of not getting an interrupt. > > Run the test 10 times and report the fastest value. > > > > You'll then find the results are entirely unstable because the cpu clock > > frequency keeps changing. > > And long enough buffers can get limited by the d-cache loads. > > > > For something as slow as AES you can count the number of cpu cycles for > > a single call and get a reasonably consistent figure. > > That will tell you whether the loop is running at the speed you might > > expect it to run at. > > (You need to use data dependencies between the start/end 'times' and > > start/end of the code being timed, x86 lfence/mfence are too slow and > > can hide the 'setup' cost of some instructions.) > > Thanks a lot for your feedback. I tried a few of your ideas and it turns out, > that they work quite well. First of all, with a single-block aes > encrypt/decrypt in our hardware (CPACF), we're very close to the resolution of > our CPU clock. > > Disclaimer: The encryption/decryption of one block takes ~32ns (~500MB/s). > These numbers should be taken with some care, as on s390 the operating system > always runs virtualized. In my test environment, I also only have access to a > machine with shared CPUs, so there might be some negative impact from other > workload. The impact of other workloads is much less likely for a short test, and if it does happen you are likely to see a value that is abnormally large. > The benchmark loops for 100 iterations now without any warm-up. In each > iteration, I measure a single aes_encrypt()/aes_decrypt() call. The lowest > value of these measurements is takes as the value for the bandwidth > calculations. Although it is not necessary in my environment, I'm doing all > iterations with preemption disabled. I think, that this might help on other > platforms to reduce the jitter of the measurement values. > > The removal of the warm-up does not have any impact on the numbers. I'm not sure what the 'warm-up' was for. The first test will be slow(er) due to I-cache misses. (That will be more noticeable for big software loops - like blake2.) Change to test parameters can affect branch prediction but that also only usually affects the first test with each set of parameters. (Unlikely to affect AES, but I could see that effect when testing mul_u64_u64_div_u64().) The only other reason for a 'warm-up' is to get the cpu frequency fast and fixed - and there ought to be a better way of doing that. > > Just for information: I also tried to measure the cycles with the same > results. The minimal measurement value of a few iterations is much more stable > that the average over a larger number of iterations. My userspace test code runs each test 10 times and prints all 10 values. I then look at them to see how consistent they are. > I also did some tests with IRQs disabled (instead of only preemption), but the > numbers stay the same. So I think, it is save enough to stay with disables > preemption. I'd actually go for disabling interrupts. What you are seeing is the effect of interrupts not happening (which is likely for a short test, but not for a long one). > > I also tried you idea, first to do a few measurements and if they are fast > enough, increase the number of iterations. But it turns out, that this it not > really necessary (at least in my env). But I can add this, it it makes sense > on other platforms. The main reason for doing that is reducing the time the tests take on a system that is massively slower (and doing software AES). Maybe someone want to run the test cases on an m68k :-) David