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Thu, 9 Jul 2026 21:58:20 +0000 (GMT) From: Shrikanth Hegde To: linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, yury.norov@gmail.com, kprateek.nayak@amd.com, iii@linux.ibm.com, corbet@lwn.net Cc: sshegde@linux.ibm.com, tglx@kernel.org, gregkh@linuxfoundation.org, pbonzini@redhat.com, seanjc@google.com, vschneid@redhat.com, huschle@linux.ibm.com, rostedt@goodmis.org, dietmar.eggemann@arm.com, maddy@linux.ibm.com, srikar@linux.ibm.com, hdanton@sina.com, chleroy@kernel.org, vineeth@bitbyteword.org, frederic@kernel.org, arighi@nvidia.com, pauld@redhat.com, christian.loehle@arm.com, tj@kernel.org, tommaso.cucinotta@gmail.com, maz@kernel.org, rafael@kernel.org, rdunlap@infradead.org, kernellwp@gmail.com, linux-doc@vger.kernel.org Subject: [PATCH v7 11/12] virt/steal_monitor: Act on steal time periodically and decide on preferred CPUs Date: Fri, 10 Jul 2026 03:26:47 +0530 Message-ID: <20260709215648.1246821-12-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260709215648.1246821-1-sshegde@linux.ibm.com> References: <20260709215648.1246821-1-sshegde@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=VebH+lp9 c=1 sm=1 tr=0 ts=6a50198a cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=VnNF1IyMAAAA:8 a=5Qv5l-8T5Df0war76qYA:9 X-Proofpoint-GUID: EglhBO0G2AkWkodFw_y3JVJDjNKzLc3_ X-Proofpoint-ORIG-GUID: bqd_C0qUZ53sCjpDWFlX4Bnccp6QveoV X-Proofpoint-Spam-Info: AW1haW4tMjYwNzA5MDIxOCBTYWx0ZWRfX7vfxFjJev0wg UwjPKs1YQZR1wgOqfrOYRoiiLHCYDCRGT6F8Iu+7alRtMdbk1O1bxKf3NWmmZe8ptrBD0/ywePj C/Etp/D+uwVu9xLNiGGK4nOVZZkptxY= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzA5MDIxOCBTYWx0ZWRfXwaOA2H6/5RD4 nc6s4f+skklD+LUfLXRzWhBLoyXsLvxCRlt9M7s4thpxxAH6YOiiv0qpotLjAoXJBZUVPao5emi bYcfKgGzImgATbuF70Z6PemGMm0kk0Fjsz7E9Xgh8zEohPJqzgd6v2OngUsjY2zMrD66/JGPF4l Jf9CG5AotsJTCOIQwHHEQBUFmk8S9ScfFzlm0wtmeD/njEVJevoWuMu63Ww8Rz0k8HKah5glM+9 GxtP4qi2e0ndgR8is5l6sUtfz/OXBmUo6F2vw6CIqRrG8A1Hiknc0WHktkvGEX03Z669f+QuyeS +dJbX1iCGFlgKmbVP8kxBejeGZy2kNZ7p4q9OpAAUS8F71/HL09d+W+ynLPPeqv4ItODjt5C643 PYaIFaFM1hbdszt/sJq2v68WpNJIiOLd503oJtrw5i9qHMHc7si9U5UKG7enBsSFHOUHZyEeb6J MHg3bG99qFmJqIAc8pQ== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-07-09_04,2026-07-09_04,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 suspectscore=0 clxscore=1015 malwarescore=0 impostorscore=0 phishscore=0 lowpriorityscore=0 adultscore=0 spamscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607090218 schedule work at regular intervals. Interval is determined by interval_ms parameter. schedule_delayed_work is used since interval_ms is usually in order of milliseconds. Work need not happen instantly. Periodic work function essentially does: - Calculate the steal_ratio as below. steal_ratio = (delta_steal * 100*100)/(delta_ns * num_cpus()) It is calculated to consider the fractional values of steal time. I.e 10 means 0.1% steal time. A few tricks such as divide by 10,000 are used to avoid possible overflow. - If steal value is higher than high threshold, call the method to reduce the preferred CPUs. - If steal value is lower or equal to low threshold, call the method to increase the preferred CPUs. - If the steal value is in between, no action is taken. - Save the values for next delta calculations. - Save the current direction of steal values to avoid oscillations. So two consecutive values of high values or low values are taken for decrease/increase of preferred CPUs. - Ensure design checks are met. 1. At least one core/CPU must be there in preferred mask. 2. preferred CPUs is subset of active CPUs. Signed-off-by: Shrikanth Hegde --- v6->v7: - Merge two patches which did periodic work function. - Misc checks for early firing, requeue work, math safety. drivers/virt/steal_monitor/sm_core.c | 76 +++++++++++++++++++++++++++- drivers/virt/steal_monitor/sm_core.h | 1 + 2 files changed, 76 insertions(+), 1 deletion(-) diff --git a/drivers/virt/steal_monitor/sm_core.c b/drivers/virt/steal_monitor/sm_core.c index 4a03c14337be..09a5c3a299c3 100644 --- a/drivers/virt/steal_monitor/sm_core.c +++ b/drivers/virt/steal_monitor/sm_core.c @@ -20,6 +20,12 @@ struct steal_monitor sm_core_ctx = { .low_threshold = 200, /* 2% */ }; +enum sm_direction { + SM_DIR_INCREASE = -1, + SM_DIR_NONE = 0, + SM_DIR_DECREASE = 1, +}; + static int param_set_interval_ms(const char *val, const struct kernel_param *kp) { unsigned int interval; @@ -106,14 +112,82 @@ module_param_cb(low_threshold, &low_threshold_ops, &sm_core_ctx.low_threshold, 0 MODULE_PARM_DESC(low_threshold, "Low steal threshold. default: 200 i.e 2%. Must be < high_threshold"); +static void compute_preferred_cpus_work(struct work_struct *work) +{ + u64 curr_steal, delta_steal, delta_ns, steal_ratio; + ktime_t now; + + now = ktime_get(); + delta_ns = ktime_to_ns(ktime_sub(now, sm_core_ctx.prev_time)); + + if (unlikely(delta_ns < NSEC_PER_MSEC)) { + pr_err_ratelimited("steal_monitor: work scheduled too soon delta_ns: %llu\n", + delta_ns); + goto requeue_work; + } + + curr_steal = get_system_steal_time(); + delta_steal = curr_steal > sm_core_ctx.prev_steal ? + curr_steal - sm_core_ctx.prev_steal : 0; + + /* Update for next calculation */ + sm_core_ctx.prev_steal = curr_steal; + sm_core_ctx.prev_time = now; + + /* + * steal_ratio = (delta_steal * 100*100)/(delta_ns * num_cpus()) + * To avoid possible overflow, divide the denominator early. + * Note minimum interval is 10ms. + */ + delta_ns = div_u64(delta_ns * get_num_cpus_steal_ratio(), 100 * 100); + steal_ratio = div64_u64(delta_steal, delta_ns); + + if (sm_core_ctx.prev_direction == SM_DIR_DECREASE && + steal_ratio > sm_core_ctx.high_threshold) + decrease_preferred_cpus(&sm_core_ctx); + if (sm_core_ctx.prev_direction == SM_DIR_INCREASE && + steal_ratio <= sm_core_ctx.low_threshold) + increase_preferred_cpus(&sm_core_ctx); + + /* + * mark the direction. Increasing the gap between hi and lo_threshold + * helps to avoid ping-pongs. + */ + if (steal_ratio > sm_core_ctx.high_threshold) + sm_core_ctx.prev_direction = SM_DIR_DECREASE; + else if (steal_ratio <= sm_core_ctx.low_threshold) + sm_core_ctx.prev_direction = SM_DIR_INCREASE; + else + sm_core_ctx.prev_direction = SM_DIR_NONE; + +requeue_work: + /* maintain design constructs always */ + WARN_ON_ONCE(cpumask_empty(cpu_preferred_mask)); + WARN_ON_ONCE(!cpumask_subset(cpu_preferred_mask, cpu_active_mask)); + + /* Trigger for next sampling */ + schedule_delayed_work(&sm_core_ctx.work, + msecs_to_jiffies(sm_core_ctx.interval_ms)); +} + static int __init steal_monitor_init(void) { - pr_info("steal_monitor is enabled\n"); + pr_info("steal_monitor is enabled. interval: %ums, high_threshold: %u, low_threshold: %u\n", + sm_core_ctx.interval_ms, sm_core_ctx.high_threshold, sm_core_ctx.low_threshold); + + INIT_DELAYED_WORK(&sm_core_ctx.work, compute_preferred_cpus_work); + sm_core_ctx.prev_steal = get_system_steal_time(); + sm_core_ctx.prev_time = ktime_get(); + + schedule_delayed_work(&sm_core_ctx.work, + msecs_to_jiffies(sm_core_ctx.interval_ms)); + return 0; } static void __exit steal_monitor_exit(void) { + cancel_delayed_work_sync(&sm_core_ctx.work); guard(cpus_read_lock)(); cpumask_copy(&__cpu_preferred_mask, cpu_active_mask); diff --git a/drivers/virt/steal_monitor/sm_core.h b/drivers/virt/steal_monitor/sm_core.h index ee68cd8b1944..7c7a9bced682 100644 --- a/drivers/virt/steal_monitor/sm_core.h +++ b/drivers/virt/steal_monitor/sm_core.h @@ -14,6 +14,7 @@ #include #include #include +#include struct steal_monitor { struct delayed_work work; -- 2.47.3