From 0d6526f82c3cdefcca47f73f5fc08dc6f335eac6 Mon Sep 17 00:00:00 2001 From: Tim Chen Date: Mon, 21 Sep 2026 17:37:22 -0700 Subject: sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling bug alb_break_llc() decides whether to break LLC preference during active load balance. It does so by testing that every runnable fair task on the source rq prefers its LLC: env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable But the two counters cover different sets. nr_pref_llc_running is updated in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued, so it follows queued tasks. h_nr_runnable is updated in set_delayed()/ clear_delayed() and drops delay-dequeued tasks. So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks, alb_break_llc() returns false, and active balance is free to pull a task off its preferred LLC. Active balance only moves runnable tasks, and this is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB skips the per-task test in can_migrate_task(). The runnable set is the one we want. Fix it on the counter side. A task should be counted in nr_pref_llc_running exactly while it is both queued on its preferred LLC (pref_llc_queued) and runnable (!sched_delayed). Define that membership once in task_pref_llc_runnable(), and adjust the counter only through pref_llc_running_inc()/pref_llc_running_dec() from the four sites that change either input: account_llc_enqueue(), account_llc_dequeue(), set_delayed() and clear_delayed(). Gating every update on the same predicate keeps the delay, wake and dequeue paths from double-counting or underflowing; see the comments at those sites for the ordering. nr_llc_running and sd->llc_counts are not touched and stay on queued semantics. Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC") Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/ Reported-by: Zhan Xusheng Suggested-by: Chen Yu Signed-off-by: Tim Chen Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Kayra Cizmeci Cc: # v7.2.x Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com --- Documentation/driver-api/serial/driver.rst | 105 +++++++++++++++++++++++++++++ 1 file changed, 105 insertions(+) create mode 100644 Documentation/driver-api/serial/driver.rst (limited to 'Documentation/driver-api/serial/driver.rst') diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst new file mode 100644 index 000000000..c1db6a1a6 --- /dev/null +++ b/Documentation/driver-api/serial/driver.rst @@ -0,0 +1,105 @@ +==================== +Low Level Serial API +==================== + + +This document is meant as a brief overview of some aspects of the new serial +driver. It is not complete, any questions you have should be directed to + + +The reference implementation is contained within amba-pl011.c. + + + +Low Level Serial Hardware Driver +-------------------------------- + +The low level serial hardware driver is responsible for supplying port +information (defined by uart_port) and a set of control methods (defined +by uart_ops) to the core serial driver. The low level driver is also +responsible for handling interrupts for the port, and providing any +console support. + + +Console Support +--------------- + +The serial core provides a few helper functions. This includes +decoding command line arguments (uart_parse_options()). + +There is also a helper function (uart_console_write()) which performs a +character by character write, translating newlines to CRLF sequences. +Driver writers are recommended to use this function rather than implementing +their own version. + + +Locking +------- + +It is the responsibility of the low level hardware driver to perform the +necessary locking using port->lock. There are some exceptions (which +are described in the struct uart_ops listing below.) + +There are two locks. A per-port spinlock, and an overall semaphore. + +From the core driver perspective, the port->lock locks the following +data:: + + port->mctrl + port->icount + port->state->xmit.head (circ_buf->head) + port->state->xmit.tail (circ_buf->tail) + +The low level driver is free to use this lock to provide any additional +locking. + +The port_sem semaphore is used to protect against ports being added/ +removed or reconfigured at inappropriate times. Since v2.6.27, this +semaphore has been the 'mutex' member of the tty_port struct, and +commonly referred to as the port mutex. + + +uart_ops +-------- + +.. kernel-doc:: include/linux/serial_core.h + :identifiers: uart_ops + +Other functions +--------------- + +.. kernel-doc:: drivers/tty/serial/serial_core.c + :identifiers: uart_update_timeout uart_get_baud_rate uart_get_divisor + uart_match_port uart_write_wakeup uart_register_driver + uart_unregister_driver uart_suspend_port uart_resume_port + uart_add_one_port uart_remove_one_port uart_console_write + uart_parse_earlycon uart_parse_options uart_set_options + uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change + uart_try_toggle_sysrq + +.. kernel-doc:: include/linux/serial_core.h + :identifiers: uart_port_tx_limited uart_port_tx + +Other notes +----------- + +It is intended some day to drop the 'unused' entries from uart_port, and +allow low level drivers to register their own individual uart_port's with +the core. This will allow drivers to use uart_port as a pointer to a +structure containing both the uart_port entry with their own extensions, +thus:: + + struct my_port { + struct uart_port port; + int my_stuff; + }; + +Modem control lines via GPIO +---------------------------- + +Some helpers are provided in order to set/get modem control lines via GPIO. + +.. kernel-doc:: drivers/tty/serial/serial_mctrl_gpio.c + :identifiers: mctrl_gpio_init mctrl_gpio_to_gpiod + mctrl_gpio_set mctrl_gpio_get mctrl_gpio_enable_ms + mctrl_gpio_disable_ms_sync mctrl_gpio_disable_ms_no_sync -- cgit v1.3.1