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Tue, 15 Jul 2025 16:39:27 +0200 From: To: Cc: "'Chris Packham'" , , , , , , , , Subject: task_non_contending() for fair_server leads to timer retries Date: Tue, 15 Jul 2025 16:39:25 +0200 Message-ID: <085d01dbf596$44286880$cc793980$@gmx.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable X-Mailer: Microsoft Outlook 16.0 Thread-Index: Adv1gAwhy4Ku/H1TS465FwEabttagw== Content-Language: de X-Provags-ID: V03:K1:h+k1Y1vdU2EiNAqEE40pLKuumFh4Y/GHSp1ueGF6x4clpXcEpCw AzktyXuAlKzMKeo7LW3jOKbdRZ0oBey9GQwJARAt8qKBOoj2A8ZqYHUi6IztwJQfqR7mR0J aKnObuDQjKv4w6A8neaHv21VpVuyRIn2kvL5cdyoc6sKSsww0iAfXqiXAN8J/fceLu9N3Aq C8X4SDad9DhAyxYT0At2w== X-Spam-Flag: NO UI-OutboundReport: notjunk:1;M01:P0:sBS9h1KOUyc=;SDXpmPFZKH4zCEtilSEyB+avGaJ 0PiXBVQgMfoURFbshkE35vAcBIApRrgTl7A55KYNOBHKgerNGOTm4q+n1hMEcVlxCJQe77iUh Pbn/MyQ1TWGt+KbGS6pFeoe3UmaADT0T5g1R2suwe9T2dgFGFbfHA5Ifl40LZ5SLW7D2Y7yf1 mtUTe0nw5YF2ev56hYvSFt60yPT2z905S8CQhEa1WqawKSxGjjvp39V7eYd19VeyXbvXP8Na1 jw5Ou0zOJnMFnylqVkuxhu6h5UqxfwHa1xxzqfK0Cy+Nd/eU0m3gTha1luqWcXVziL1a4mwBd frz3uHOCzCC5H91ISDgqO23D2FyOZe0xRs224JDu5E3YLbOzDOACpcvqn0y6S3t3zPHJ2FTya tCnnfbtd3BpGMu8Bs+woY3diCeI7NW4MUnAnbhrCBWOkCm/f71uc1EbxAe5F2M8mi3mXWoIUe 16hK9r61CV+c2nQ4m7ha+MALRtABP6HjgMoswmF8fduktsLDQzZHnqUwvAVfIZAAsAxacw21a SebHHkqQkARW6RvyX9h94Z/MdOZf3adO9OJT9BDeXwkCvaTucoXs4lbtG4bkKJecI+Nbt8erY Los2ltXYKUypvJuNUOM4OoawwDRstUgus87/TPfs7e8ODmN0rmOCO9r8KPD/k3M7x5qifsDW7 DNJgZ/G707ZmSsi1ZOkEd95nwPZ8R37juCZb+WJnKwBgpgdKZjbLN8RQNLup7wir/Wr/hNLSs 4Eek9L1HmMdXdA9X1vK7cgFXcr+nQpQPnkoSKhW2WmmFYwH9v3cC3BuRb4Qwe/S7NZe5EWS7A BeDBbyu+r2xAlx9AF20gFNBf4PigMD6otTdc+YxIIArveTWbQ9E+hSI/MUwptb0RlGtsFXV8Q rHzKmtkXTfb7oPwv4aR4Ree9ylGm++VWyYpgdJuFSH3uJQsYzKg2d/2uQu3JbSBJdKkeHV0ke 6l0ancS5fuLOxjPmguUd1RfFZM7R2FXOcrBZEsgrVB4aaf4LPjEWMM1X+T91sqFYKfQ3muYdb e62hZJ92qk6WzQkuHyCQihikd2CYwt8YQyEE1qID87OYC6PzJh01vVG+lywyw6tOd96uxINMl iGgwHbnerMoTLRj9HTXtGHCcMf2gUTpdxFR3Wvdcpxarw0dDdg2kkHFR3iPvJOwLZSwKRM4cy 02+EL9SVHCZ4ttToyrGoUW/PxNjvmc0L+KbmTAMp5FqtQ0WpyeXDG8jhMbBs315kyCsObSDtV dZjZS9Ryt8Idts45p9ctt8d5gN+YseiX0Xu3BQvp60597u5F7HFrxlMw4cb3TiScdSag2jUyW uFEh/6jWvAQtJSIg+CGb8YxbZLHGvID3XVGT1LXnKkHMKt/YTVI8ykp1Sg27J1VO8SCWDDnAg QAmmGy8fiA8m7uAdSV/y3DAzeOWX3EoiweHnpLWbPKvBDSTecjEJ96/VHtKG1hYPiDsFXwo+n NgNOd7aRszaaRaU5uvHAELoTqXdzpg5v/i+PXCWywOprY4abJs0dh4lrTjFwIrhetnRGAH6VB iYLhGdBYubLg0qt80zsj6S/MWYBm5QwYbak+9N+ytqm1SZUkGcFp9roxXh4kZ6IU0jXwpEvEH f8Y9eT88kjqzQzmXzwO6x8frdtobMT3BJaHDrHLJju64BEqXvNChQFzHijzCUjd9uqkwiLT8h 51aBfNnc2n78WBYENQu33t32llUsuI+XpFCdr57JUyQ5CROszAfX/Ah5V6PddMVf6F8I5YN4W fNBlI/j+nqDCuOJqvZ0n1CYMO/ZY3otKapuo8Gk0hDrv9AD/830yLOrg6x8HyOltoKKIz//gU svMt74QIq3F86BMDpBxsCujn0uBCvGs7zywLYNugE0jKqVs0TarpWAdT8Q/93/aOIZOA985iv ZW5xyWHgDxhWb+gcbQw6Eh0zSbgQMonEKMJdeyLMtxfpVfHkite+bQE2mTE2KwhoFs3EKpEd0 JCXW19VLcaMTId9+/2fUzYysV4G5dleptGIlBrt2jxboGT+PSac4FCGuBJVnrwJl3ze8rzlkE 5DGd7jytYLspzYYpMwuP21WfAQ7cE9jtBn3vF2okxZcEQ01NN0C3PESgB149lIuijAotAS23d fEjl0JJTBCPPA2LAwizIo/YWIDofp/vOvFi2yv4CYiXfpKfr+D7baB91SpPsG4GqNvqVWA662 cEFJNnzHhLFWOcaZcdeNAGhxlb59VOdUsyflKGdi0RlwlCt0B5kLJ7EoPnOohmE1oPcWCe/Br nRb9XEF/77Tf95pLeiIWDV6phiFG9mXRQzcnwvWY+TdxhKF7qor/wdmbPFGFTM7MWcRWf6B6v jiGfa5z7I7cQx9Lq1XMjhGM4I1NfkIGrcJsy+wH4UqOGoDQ7c6w7dPanTps49ss+96vRKddQd eoSPJfXbL1O2gKOnmd1jGutrUZQOwfSwitYHqggfI6jP3tcCD7ex9VAJbJ6Y28wPaqM6mbwPE pz8LxOEVxnlubz9aDb5DqtyISTTT5gBtQDl/F+PbZ7upnTo+7hBBV+mJzTo18dBq5W1RVw33K UdBXdnR4nHlsTlGbgOSMQ0M/DCkiTDJf0D07z6v+o7HyJ5Zjg1NVHvI0JQRbz7kbRj2G7IIhW HzgJ2cx+GgLdeGs++BUgyrcQhjW4NE7/A+YtkDqaC9gd+SpDq0BXrHEFwIi+X9iFdKEaAVyzX T592ts3xV9fCbSdWyFCFhixwH9TNsjwRhhB9R3Cq6A8kr9onLeY83S8nxqaPIJkupyPBZm5HD KRkEeTaTf74dkiuJL6NELGN2RY8BF8HgrDJ4cG1rHa0XGUwF2dd+Z8fQq6lBXQxtSwkLHIbXC VOOA3kkZDgnhIV1cJCgj0VtfeN80qMxHXhQ9Rq1lMv/07QL/k61nImzthBOQ9KfajxioZM09Q 9iCYzK5mFX+N99TBEprDvWu1ZCYtWmJ1ed7jqgZSpFb+QGwl4MiUJJrml5LlUXxVOkr326GxS kEv+sHXSiXnWaaFVk8nWmZoP1Z+dpZhwHfVrLVtut9u7PuAxMaw8tAvVQxaxwsmpRy6nDgkIK Fia3lLowAaeUZVod3whY/lL/L3srzKkivDZJBGlQr9bXNnV56PJ8URHcUzC4QTdv/B+QTdsFl dc4LskrKwg+zo3uewrcQA73mx8cs7SgDVObUhONtK4QWYwhc090j3S1nyavRBo5yKJp1195Ya yXukIvar1oy7i6LSIakz2Oqy9M6RasNnG0UTZ3gjRynR6Py7nDPaICqyE922JtAp78i8openz gokLh9qEkJ58JB6uZ+aXWemdleYArEUIdZhZgir+xclwjbzBmTqGPhUswZWPHW8H2Th4/FRl/ xrqX2ly+GLByO7A5VeePg7NVUfXMfHt60zZbZNP8gStIgB2AnpUJqowR63ulrITqP7rky4dLM ybBDUYUir3mK/znPGmWvIdqBrBQCqsdHboUWJChNpq3XA43ED3LyPHKjBrjjyrlasWkztb2nI UZX7rA4oJmZ5PHcpus0Ckg1zHIxDb28Fe1Qp1DuhnwJ02QK+KshyZHpNJPPO3/mORFv2QY0XN Ug== Hi Peter, I'm currently investigating issues with the timer-rtl-otto driver in=20 6.12 longterm on the Realtek MIPS switch platform (Chris is working hard to upstream this). While doing so I observed that timer retries=20 continually increase (~6/second) according to /proc/timer_list. The=20 system is otherwise totally idle. 6.6 longterm does not show that issue. I'm unsure if this is related but documentation reads like "that's bad".= =20 To be sure about this one I nailed it down to the fair server. Whenever task_non_contending() handles the fair_server, zerolag_time is calculated as 0 and a hrtimer_start(timer, 0, ...) call is issued. Going down the stack clockevents_program_event() thinks the target time has=20 been exceeded. So it instructs clockevents_program_min_delta() to set a minimum delta time (2560ns for the otto timer). From there the retry counter is increased. See attached output. To silence the noise and focus on the real bug I use this workaround in task_non_contending():=20 if ((dl_se =3D=3D &rq->fair_server) && (zerolag_time =3D=3D 0)) zerolag_time =3D 6000; Totally crap but serves the purpose. Maybe you can share insights about this (un)desired behaviour.=20 Thanks in advance. Markus # uptime 00:41:19 up 41 min, load average: 0.00, 0.00, 0.00 # cat /proc/timer_list ... Tick Device: mode: 1 Per CPU device: 0 Clock Event Device: timer@3100 max_delta_ns: 85899344321 min_delta_ns: 2560 mult: 13421773 shift: 32 mode: 3 next_event: 2469910000000 nsecs set_next_event: rttm_next_event shutdown: rttm_state_shutdown periodic: rttm_state_periodic oneshot: rttm_state_oneshot event_handler: hrtimer_interrupt retries: 14646