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b=YIw5e6wMpwqsBArxqgPqW29Eovlha+8sP/PTHAchI25czpN3OXsjCVgW7EgERL1c8 My3d17ab8a5zHMYQi8ouEaakQaf6sDq3Ru+E1qoq4aC81KWIntcvpBpN3ENlheg260 AzshRMXkeoyW75MWosBPPsriMsm2jUiS5Sk17GoyP6i9hd/HkSgtpMKUEqjtr6eRNi PGYIwJHRPwdcbNYQykyQVOdpfPVQmHvgC5yYmUdK7xem6OIjfT5InA6UuSc92vudMU +4fi066thhTqs6JDxPzKJ/zyFkzAAyaGzoz8mZW0V5PNOhQhXG0nBmo8C/awYzhZ8u Zgb6fYSxrKMPg== From: SeongJae Park To: Josh Law Cc: SeongJae Park , akpm@linux-foundation.org, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: (sashiko review) [PATCH] mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses() Date: Wed, 25 Mar 2026 07:43:46 -0700 Message-ID: <20260325144347.87818-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: hswiharp4zjptkkuospq6yo6poyakobs X-Rspamd-Queue-Id: 7133310001D X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1774449829-643520 X-HE-Meta: U2FsdGVkX19SfqWKi7AAFp7lrqblJGdFoegUjeT1parB0+dqplBtFfVorUEk+wWW/IJ3+xu0Y2uHTx6duCnA693c8iWouDotI0wJN0GXIq8HI0rSUnDoOZQN/3EPf6fP0dgBoAbFnGXrrTIzJyrJEdY2IsrqBeZiF1xuK2N1h6AmbAMLz8M58xGNDJCtfljvqiKJzGTdzHrJMJCK0lXckb39SF0sZKKjX4g323GzElM1vL5DvgaAPnBJmJ53c1YnQKJ8p4L9BdOGdrsLedZPglmISKtyVuOSoVe9BUfGz/tdJF3nqtCLxs//fdWbv0uDcxkA6uNEibHEl+5S+jmR9GewLmzfXDxF8b2Iu2Gk1xqtYR6qwdXd97uf5+gJ/TrSX8oQQp4J2IJhEgYqxqjW7ODfiGGGSiuhZcKiz2cjMiT0FvcBMD5OmuZn0uvpm9YRHlCHBfBitgEbFO8LLjQd8KZ3q0qe4y1rqrfEHa6IJZCxxoIZZwlIp0xcco9v7fCHhqJL57PgPSexdVoOzjLzqT91YpLDhqaBCxbuAotPjbJCItIDMVFOGU55mpCv4zXx/cFUCIlQmKsClvqtTS6VQYEug9d5RKRo8hdEOsKWm4jikF7ynhmzyJsktxOletEkD5twycycUA83uoeYvBiXd1ZRn8EK4YeikWB15zA5lZKN2AlkiKuxNQn9gxb6Xw3k0bbmXhPY++33O5nfqsKM2uAmtbk3Jvl54PtugA9y0aCjuYEZNovljiLnAMiD8ARTxo4iiozRdo/5tDOb+GQ1rXKutxtJB+C6EAf2ZZ59EQSZ8JTYwBj1PAO+2MMQosr4qp4CR1SZ3vBI/6BTbTvKFvmv+M1kfzCZ/gkBRgbkL1UjwB3NSS5YDIdvufmKA2wcHXCW+h3euRI9Mxch4KLEq581sjj19tV+Wnfn8/W7wkwgs8ZG5k2e4wE0fd7FIJMC1sIF+Bch/rUjVsb+uko SrVegVy7 C4ctEQ6K0Jby1q1VvllVAjc4bdE/ZVDmHUHJcvMleHCaSjP2YxDSZZdOB8gMiz/WeuCzVZfii2yuBlceEHCzGIW0PdTEssbz+pHnUEwOeYmqYZYCR49gL0L0zoF+I1Iv90mY6pcL9C+5LJQW0DFpEDu4dF0pH7No0dXg1BO6j+6qububb/+iidhfiE+Wwq1QwBnXEU/c90WGgq0gAGvdyywd3pmKA8kfgdE7O0wEYiGEf+Kb1HBKaQanSwqTXXh7dQPFW+Mr1pZbtrIlBp8Jax4kk7ZZjnu3fMb6gu+DnruYJUSacP12w67d5qhR2+oftpO+cCS72BGBbP7iaLviuNTuT0mc8WNk9dVCcQJnrIhp67TCR1KUdiFRsr/geAWgoDwMNZbqYsMWNYWERRKSRYpEg1fTjGtGxelJ9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 25 Mar 2026 07:36:20 +0000 Josh Law wrote: > > > On 25 March 2026 01:16:52 GMT, SeongJae Park wrote: > >Forwarding Sashiko review for doing discussions via mails. > > > ># review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org > ># start of sashiko.dev inline review > >commit ff5e227a18cd69435749b0aae0dc2c762c4f0a47 > >Author: Josh Law > > > >mm/damon/core: eliminate hot-path integer division in damon_max_nr_accesses() > > > >Hardware integer division is slow. The function damon_max_nr_accesses(), > >which is called very frequently, performs an integer division. > >However, the struct damon_attrs already caches this exact ratio in the > >internal field aggr_samples. We can eliminate the hardware division in > >the hot path by simply returning aggr_samples. > > > >> diff --git a/include/linux/damon.h b/include/linux/damon.h > >> index d9a3babbafc1..04c8a052fcfb 100644 > >> --- a/include/linux/damon.h > >> +++ b/include/linux/damon.h > >> @@ -978,8 +978,7 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx) > >> static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs) > >> { > >> /* {aggr,sample}_interval are unsigned long, hence could overflow */ > >> - return min(attrs->aggr_interval / attrs->sample_interval, > >> - (unsigned long)UINT_MAX); > >> + return min_t(unsigned long, attrs->aggr_samples, UINT_MAX); > >> } > > > >If an administrator dynamically updates aggr_interval via sysfs and commits > >the inputs, will attrs->aggr_samples be correctly recalculated? > > > >If aggr_samples becomes stale, kdamond_fn() will compute the real aggregation > >loop count and allow r->nr_accesses to increment up to the new bounds (for > >example, 80 loops). Meanwhile, damon_max_nr_accesses() will return the > >stale aggr_samples cache (for example, 20). > > > >When evaluating schemes, damon_hot_score() normalizes r->nr_accesses against > >this stale maximum, producing a freq_subscore that severely exceeds > >DAMON_MAX_SUBSCORE. This causes the calculated hotness to exceed > >DAMOS_MAX_SCORE. > > > >For DAMOS_PAGEOUT actions used by damon_reclaim, damon_cold_score() returns > >DAMOS_MAX_SCORE - hotness, yielding a highly negative score. > > > >Could this negative score cause an out-of-bounds write on the > >kernel heap when used as an array index? > > > >mm/damon/core.c:damos_walk() { > > ... > > c->regions_score_histogram[score] += damon_sz_region(r); > > ... > >} > > > >> diff --git a/mm/damon/core.c b/mm/damon/core.c > >> index db6c67e52d2b..afb058f7da5c 100644 > >> --- a/mm/damon/core.c > >> +++ b/mm/damon/core.c > >> @@ -587,6 +587,7 @@ struct damon_ctx *damon_new_ctx(void) > >> ctx->attrs.sample_interval = 5 * 1000; > >> ctx->attrs.aggr_interval = 100 * 1000; > >> ctx->attrs.ops_update_interval = 60 * 1000 * 1000; > >> + ctx->attrs.aggr_samples = 20; > > > >Since this value is cached here, but not updated when intervals change, does > >it remain permanently stale during dynamic updates? > > > > > ># end of sashiko.dev inline review > ># review url: https://sashiko.dev/#/patchset/20260324154005.83651-1-objecting@objecting.org > ># > ># hkml [1] generated a draft of this mail. It can be regenerated > ># using below command: > ># > ># hkml patch sashiko_dev --for_forwarding \ > ># 20260324154005.83651-1-objecting@objecting.org > ># > ># [1] https://github.com/sjp38/hackermail > > > >Sent using hkml (https://github.com/sjp38/hackermail) > > > > So it's nacked? Or is it just a review Just a review from an AI :) I treat sashiko review as just another review comments. I'm forwarding these in this way for a case that someone is interested in the comments and want to share some findings from those in a way easy for mailing list based communication. Thanks, SJ [...]