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Fri, 31 Oct 2025 08:41:24 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGzEJR5rAki79Ma8q+7kkmBdMIWsBwJsRMW2wtEaLva07Vg+1OMiXKWsCQUIlAiQZR5f1VJWg== X-Received: by 2002:a05:600c:3489:b0:471:15bb:ad7f with SMTP id 5b1f17b1804b1-477307e1f05mr36743515e9.17.1761925283935; Fri, 31 Oct 2025 08:41:23 -0700 (PDT) Received: from gmonaco-thinkpadt14gen3.rmtit.csb ([185.107.56.30]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4773c5647e1sm2162025e9.15.2025.10.31.08.41.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Oct 2025 08:41:23 -0700 (PDT) Message-ID: <2daa2e6217eeaa239616303626c0d73d808ae947.camel@redhat.com> Subject: Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline From: Gabriele Monaco To: Peter Zijlstra Cc: Juri Lelli , linux-kernel@vger.kernel.org, Ingo Molnar , Clark Williams , arighi@nvidia.com Date: Fri, 31 Oct 2025 16:41:22 +0100 In-Reply-To: <20251031152005.GT3245006@noisy.programming.kicks-ass.net> References: <20251014102541.GS3245006@noisy.programming.kicks-ass.net> <83a5971ef07226737421737f889795ec57b6fa6c.camel@redhat.com> <20251014193300.GA1206438@noisy.programming.kicks-ass.net> <20251020141130.GJ3245006@noisy.programming.kicks-ass.net> <8dc29e28a4d87954378ef1d989e0374526b44723.camel@redhat.com> <20251030184205.GB2989771@noisy.programming.kicks-ass.net> <20251031130543.GV4068168@noisy.programming.kicks-ass.net> <1f2ad071e59db2ed8bc0b382ae202b7474d07afc.camel@redhat.com> <20251031152005.GT3245006@noisy.programming.kicks-ass.net> Autocrypt: addr=gmonaco@redhat.com; prefer-encrypt=mutual; keydata=mDMEZuK5YxYJKwYBBAHaRw8BAQdAmJ3dM9Sz6/Hodu33Qrf8QH2bNeNbOikqYtxWFLVm0 1a0JEdhYnJpZWxlIE1vbmFjbyA8Z21vbmFjb0BrZXJuZWwub3JnPoiZBBMWCgBBFiEEysoR+AuB3R Zwp6j270psSVh4TfIFAmjKX2MCGwMFCQWjmoAFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgk Q70psSVh4TfIQuAD+JulczTN6l7oJjyroySU55Fbjdvo52xiYYlMjPG7dCTsBAMFI7dSL5zg98I+8 cXY1J7kyNsY6/dcipqBM4RMaxXsOtCRHYWJyaWVsZSBNb25hY28gPGdtb25hY29AcmVkaGF0LmNvb T6InAQTFgoARAIbAwUJBaOagAULCQgHAgIiAgYVCgkICwIEFgIDAQIeBwIXgBYhBMrKEfgLgd0WcK eo9u9KbElYeE3yBQJoymCyAhkBAAoJEO9KbElYeE3yjX4BAJ/ETNnlHn8OjZPT77xGmal9kbT1bC1 7DfrYVISWV2Y1AP9HdAMhWNAvtCtN2S1beYjNybuK6IzWYcFfeOV+OBWRDQ== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2 (3.56.2-2.fc42) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2025-10-31 at 16:20 +0100, Peter Zijlstra wrote: > On Fri, Oct 31, 2025 at 02:24:17PM +0100, Gabriele Monaco wrote: > >=20 > > Different scenario if I have the CPU busy with other tasks (e.g. RT > > policies), there I can see the server stopping and starting again. > > After I do this I seem to get a different behaviour (even some boosting > > after idle), I'm trying to understand what's going on. > >=20 After running some heavy RT workload (stress-ng --cpu 10 --sched rr) I do s= ee the server stopping and starting as the models would expect, but somehow it= 's always boosting as soon as it's started. Apparently dl_defer_running is always 1 in that scenario. Perhaps running i= dle counts as running something too, so it never defers. But I can't really see= how this happens.. -0 12.964637: (+8) sched_switch: swapper/9:0 [120] R = =3D=3D> irq/50:212 [49] irq/50-212 12.964699: (+58) sched_wakeup: trace-cmd:544 [120] = CPU:000 irq/50-212 12.964831: (+24) sched_migrate_task: comm=3Dtrace-cmd pid= =3D544 prio=3D120 orig_cpu=3D0 dest_cpu=3D9 irq/50-212 12.964842: (+11) event_nomiss: -9: idle x dl_server= _start -> idle irq/50-212 12.964843: (+1) sched_dl_server_start: comm=3Dserver pid= =3D-9 runtime=3D16238881 deadline=3D13592407587 yielded=3D0 irq/50-212 12.964854: (+11) bprint: pick_task_dl: Pickin= g server 9 (armed 1 running 1) irq/50-212 12.964861: (+7) event_nomiss: -9: idle x sched_swi= tch_in -> running irq/50-212 12.964862: (+1) sched_switch: irq/50:212 [49] S = =3D=3D> trace-cmd:544 [120] The trace-cmd in the example above is boosted, like highlighted by that bpr= int=09 just before. > Ooooh, because idle time is accounted against the server budget too. >=20 > That is, idle and running fair are both [2] in my comment. So we never > get to [5]. Yeah that must be it, however I don't think it explains the other curious behaviour above. > Humm, let me noodle a bit more on this. Also, I should see if I can get > graphviz to draw ascii art state diagrams :-) Dot doesn't do it but graph-easy does reasonably well. Thanks, Gabriele