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Wed, 8 Jul 2026 16:55:10 +0000 (GMT) Message-ID: <7eea685e-e5ab-4487-bafb-3131969e5acd@linux.ibm.com> Date: Wed, 8 Jul 2026 22:25:09 +0530 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 20/23] virt/steal_monitor: Act on steal values at regular intervals To: Yury Norov Cc: 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, 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 References: <20260701141654.500125-1-sshegde@linux.ibm.com> <20260701141654.500125-21-sshegde@linux.ibm.com> <6548b668-d3e1-42dd-be9f-2fe3193874a1@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=M7J97Sws c=1 sm=1 tr=0 ts=6a4e80fb cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=7zOuVmdEg6JVUKogFm4A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzA4MDE2NSBTYWx0ZWRfX/3bBwzDrqaoU vXvktXkE6DimdEudmPRn93OHcO6kOfphSqTAfKSi3woHD9/Ep73gIA/aAJiNfCYKbfhW4ydeFCp leNU6C3KcQfKrc4TIjpK/hXA6CIe0TT/sUogUtJ2NVWd77bzJPpr91XVV49WYaXQU5krRcx545F 92L5RdlVdZczAUzDYjttcgO0spL3/8aUpb8CTASU5BNNJ+BoGTovw+P8S5vQTS5O3ei72UIl9Il xXA19igVFq6Kq+nklcIvn8xqLLM7iwuMAxX08ZGmspODQyN6ycxVwVyMFonO8PU8N8VKa8KU1eJ q9TyhXZRMNT30LlBaDDYb2iDuRd6ZEYOgV5TtlgawwQPUQXh/TTFzCt+Wqtpw21RkLKWiurVWcn 82nUq05jeV8WtYqdaDN4a1vf9I+8L9Y1o8ROKYchQtPEtws+p3L5xT8tiJnzxkYmBZ/XVYwLr+d +B9lLoosKODE6JJ1uXQ== X-Proofpoint-Spam-Info: AW1haW4tMjYwNzA4MDE2NSBTYWx0ZWRfX/hiejhVWAxf2 +buPAH7tGStor76r6qWr86wqERO4zwKEWh3fmeJKxVgm8k2Qler3Rree0iwAFnvOp3X56ONLlKE HCMVEFsiUTi9NXz11eu44WwXNCzodEU= X-Proofpoint-GUID: hUW7vE16ZPD1iH0LbQPFMMRW_umJxuYb X-Proofpoint-ORIG-GUID: vgFdX69fPlUH-SEKP9-2wxsdrQI3Atky 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=2026-07-08_03,2026-07-08_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 lowpriorityscore=0 malwarescore=0 clxscore=1015 adultscore=0 priorityscore=1501 bulkscore=0 spamscore=0 impostorscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607080165 On 7/8/26 9:43 PM, Yury Norov wrote: > On Wed, Jul 08, 2026 at 04:03:18PM +0530, Shrikanth Hegde wrote: >> >> >> On 7/7/26 12:46 PM, Shrikanth Hegde wrote: >>> >>> >>> On 7/7/26 2:03 AM, Yury Norov wrote: >>>> On Wed, Jul 01, 2026 at 07:46:51PM +0530, Shrikanth Hegde wrote: >>>>> This is the steal_monitor core functionality done in periodic work >>>>> >>>>> - Calculate the steal_ratio. It is multiplied by 100 to consider the >>>>>    fractional values of steal time. I.e 10 means 0.1% steal time. >>>>> - 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. >>>>> >>>>> Signed-off-by: Shrikanth Hegde >>>>> --- >>>>> v5->v6: >>>>> - Address u64 overflow concerns. >>>>> >>>>>   drivers/virt/steal_monitor/sm_core.c | 33 ++++++++++++++++++++++++++++ >>>>>   1 file changed, 33 insertions(+) >>>>> >>>>> diff --git a/drivers/virt/steal_monitor/sm_core.c >>>>> b/drivers/virt/ steal_monitor/sm_core.c >>>>> index b499faa61010..7b7435f79b85 100644 >>>>> --- a/drivers/virt/steal_monitor/sm_core.c >>>>> +++ b/drivers/virt/steal_monitor/sm_core.c >>>>> @@ -34,6 +34,37 @@ MODULE_PARM_DESC(low_threshold, >>>>>   static void compute_preferred_cpus_work(struct work_struct *work) >>>>>   { >>>>> +    u64 curr_steal, delta_steal, delta_ns, steal_ratio; >>>>> +    ktime_t now; >>>>> + >>>>> +    curr_steal = get_system_steal_time(); >>>>> +    now = ktime_get(); >>>>> + >>>>> +    /* get the deltas */ >>>>> +    delta_steal = curr_steal > sm_core_ctx.prev_steal ? >>>>> +              curr_steal - sm_core_ctx.prev_steal : 0; >>>>> +    delta_ns = max_t(u64, ktime_to_ns(ktime_sub(now, >>>>> sm_core_ctx.prev_time)), 1); >>>> >>>> The below return on '!delta_ns' makes this max(...) useless, right? >>>> Regardless, if the time between 2 measures is less then 1ns, I >>>> believe, the whole measure is not trustworthy >>>> >>> >>> Seeing sashiko comment on divide by zero, i forgot I had max above there :( >>> >>>>> + >>>>> +    /* Update for next calculation */ >>>>> +    sm_core_ctx.prev_steal = curr_steal; >>>>> +    sm_core_ctx.prev_time = now; >>>> >>>> So below return should go prior to this update, because 'now' is actually >>>> the same as 'prev_time', right? >>>> >>>> I don't understand why 'now' can be so close to prev_time, because >>>> you've scheduled this callback on the regular interval. But if that's >>>> possible, can you explain that and do like this at the very beginning >>>> of the function: >>>> >> >> I don't think it is possible. >> >>>>          now = ktime_get(); >>>>          if (unlikely(now < sm_core_ctx.prev_time + >>>> sm_core_ctx.interval / 2)) { >>>>                  pr_warn(...); >>>>                  return; >>>>          } >>> >>> Pretty much, just have to requeue the work. >>> >>>> >>>> And if it's a never-happen condition, just use WARN_ON(). >>>> >> >> I have decided keep just max_t() to ensure no divide by zero happens. >> >> Issue with putting sm_core_ctx.interval_ms check is that, one may edit >> it while the work is queued, but not yet called. It would print wrongly >> in that case. > > Who is that 'one'? If it's userspace, you can simply disallow changing > the interval. If it's kernel, you're already compromised, and > everything is not trustworthy. > > I would simply disallow to change any parameters while the driver is > loaded. If I need to change something - I unload it and load with new > interval, or whatever. It is user writing to /sys/module/steal_monitor/parameters/interval_ms that i thought of. So user. If we disable then those checks can be valid. > >> Even if work function gets called prematurely, it should be fine since the >> steal also increases accordingly. No issues in delta logic. >> >> So capping to 1 to ensure no divide by 0 happens due to scaling of >> delta_ns is good enough. > > If there's 1 nanosecond between two calls, your statistics is very > inaccurate, pretty much a noise. You want to make decision based on > noise. That's simply wrong. yes, that makes sense too. I have kept interval_ms/8 as safe bet. Return will not queue the timer and effectively disables it. ktime_t now; now = ktime_get(); delta_ns = ktime_to_ns(ktime_sub(now, sm_core_ctx.prev_time)); if (unlikely(delta_ns < sm_core_ctx.interval_ms * ((u64)NSEC_PER_MSEC >> 3))) { pr_err("work scheduled too soon delta_time: %llu ns. Abort steal_monitor.\n", delta_ns); guard(cpus_read_lock)(); cpumask_copy(&__cpu_preferred_mask, cpu_active_mask); return; } > >>> Ya, it is a never-happen condition. I will use WARN_ON. >>>