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Mon, 12 Jan 2026 18:49:01 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4DCE420043; Mon, 12 Jan 2026 18:49:01 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 86AAB20040; Mon, 12 Jan 2026 18:48:58 +0000 (GMT) Received: from [9.124.216.1] (unknown [9.124.216.1]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 12 Jan 2026 18:48:58 +0000 (GMT) Message-ID: Date: Tue, 13 Jan 2026 00:18:57 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] sched/deadline: Log Fair Server re-enablement for symmetry with debugfs To: Aaron Tomlin , K Prateek Nayak Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, neelx@suse.com, sean@ashe.io, mproche@gmail.com, linux-kernel@vger.kernel.org References: <20260109031959.2786873-1-atomlin@atomlin.com> <20260109031959.2786873-2-atomlin@atomlin.com> <0e76c836-c645-4fd0-9d86-b47b8834eac2@amd.com> <4b3cabf1-76ff-4d1a-be1f-ab3fe3bfd935@amd.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=DI6CIiNb c=1 sm=1 tr=0 ts=69654220 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=bzHENX6xAK47jOmXHkQA:9 a=QEXdDO2ut3YA:10 a=CpV4FHq4P1MA:10 X-Proofpoint-GUID: AE9R0PW8XWp1cRnSvx70z3O5nohqfJBC X-Proofpoint-ORIG-GUID: sqwgZ8z1-Jt9MjRTmw7CfZyBcE70xQKW X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEyMDE1MSBTYWx0ZWRfX0/DVUR86DTdx T4CwHk9orggH80zt8/njH22JFJn2U+latlxeBull2uv96m1au+Vkg2ieGilveb/OCkXgo6RaC29 qQzVGSalcR1Qk7+rsUtdYMymZDsx82+l8tX6E+iWxM5jk0razbwe/1G+fer3TJ2MKqOHFNKDiG/ X6rO1XsYP4qC8pjg0aWrDDvr+Sox8zhJc8Thg8tPLvPfb4biUO4SG6h+ttfZL8O7hB9QOICJrWa hkDv0pRy7xL3LmYSoUMr67YtaXmivZxmieJBRY8eB0CsFqur3UxQBCwPKJIZx+Il6XcO6l8k6dA +Q/jPCg7LGV3aP4EREOG7fobxgbezfBzJzjErZkuqdSZnA8z6scr5T3NAO3dLoCA/bPk/Y7rpt/ dUf9rwbh/wkAApWsR+kVIkMPNio/5ZjKO6z5ONCXnNC8iyit61GcnQNOiWCRFZuLwN6Ne+ZY9GJ ZvFpqTrskq8lH1uOZkw== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-12_05,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 priorityscore=1501 lowpriorityscore=0 adultscore=0 malwarescore=0 spamscore=0 suspectscore=0 phishscore=0 impostorscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601120151 On 1/12/26 8:02 PM, Aaron Tomlin wrote: > On Mon, Jan 12, 2026 at 10:44:03AM +0530, K Prateek Nayak wrote: >> I believe the suggested solution to that was to trace the reason for the >> kthread/fair task waking up on isolated CPUs and prevent the wakeup if >> it is for some unnecessary operation as opposed to disabling the fair >> server. > > Hi Prateek, > >> We have tools like https://docs.kernel.org/trace/osnoise-tracer.html to >> capture these noise. Trace the noise, bring up the case where isolation >> is broken on the current *upstream* kernel to the mailing list, and we >> can solve it for everyone instead of disabling fair server as a duct >> tape. > > Thank you for your insights. > > I fully concur that, in an ideal world, the "correct" solution is > invariably to identify and eliminate the root cause of any spurious > SCHED_NORMAL wakeups on isolated CPUs. Tools such as the osnoise tracer are > indeed invaluable for this pursuit. > > However, I would respectfully submit that there remains a distinction > between the theoretical purity of the kernel and the pragmatic reality of > managing highly specialised, latency-critical partitions. > > It is pertinent to note that the kernel currently affords users the > capability to manually modify the Fair Server's parameters via > /sys/kernel/debug/sched/fair_server/. As this resides within debugfs, it > is, by definition, a debug-only interface and not strictly considered > "production safe" or guaranteed to be free from side effects. The capacity > for a user to destabilise their system via this interface - effectively > "shooting themselves in the foot" - already exists. This existing interface > is useful for educated users who are willing to accept full accountability > for system stability in exchange for absolute determinism for a defined > period of time. > >> Juri, Peter, is changing the fair server's bandwidth frequently very >> common scenario is the field? >> >> If not, can we add a pr_warn() for when the fair server's parameters >> are changed by the userspace just to catch any absurd values that >> reduce the bandwidth to a minimum without disabling the server? >> >> I can do something absolutely stupid like this without dmesg logging >> anything that would indicate I'm being stupid: >> >> # echo 4000000000 > /sys/kernel/debug/sched/fair_server/cpu0/period >> # echo 1 > /sys/kernel/debug/sched/fair_server/cpu0/runtime >> # sudo taskset -c 0 chrt -r 99 ~/scripts/loop& >> # taskset -c 0 bash -c 'mkdir /sys/fs/cgroup/cg0; echo $$ > /sys/fs/cgroup/cg0/cgroup.procs;' >> >> ... wait for a while >> >> INFO: task bash:4272 blocked for more than 120 seconds. >> Not tainted 6.19.0-rc1-tip+ #162 >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >> task:bash state:D stack:0 pid:4272 tgid:4272 ppid:4271 task_flags:0x400100 flags:0x00080000 >> >> >> A taint might be too far but a log should be acceptable? > > Regarding your valid concern about visibility and safety: I am agreeable to > hardening the observability of such changes. In the next iteration, I > propose to introduce a pr_warn() that triggers whenever the Fair Server's > runtime or period is modified from its default value (50 * NSEC_PER_MSEC > and 1000 * NSEC_PER_MSEC). This will ensure that any deviation - whether it > be a complete disablement or a reduction to unsafe levels - is clearly > logged, rightfully alerting administrators to the non-standard > configuration without removing the latitude required by those who > explicitly need to make that trade-off. > Currently it is 5%. It is going to be tricky to define unsafe levels. Looks like Either one wants it or don't want interference from it. Are there any users changing the default value?