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/admin-guide/device-mapper/delay.rst | 54 +++++++++++++++++++++++ 1 file changed, 54 insertions(+) create mode 100644 Documentation/admin-guide/device-mapper/delay.rst (limited to 'Documentation/admin-guide/device-mapper/delay.rst') diff --git a/Documentation/admin-guide/device-mapper/delay.rst b/Documentation/admin-guide/device-mapper/delay.rst new file mode 100644 index 000000000..a1e673c0e --- /dev/null +++ b/Documentation/admin-guide/device-mapper/delay.rst @@ -0,0 +1,54 @@ +======== +dm-delay +======== + +Device-Mapper's "delay" target delays reads and/or writes +and/or flushes and optionally maps them to different devices. + +Arguments:: + + [ + [ ]] + +Table line has to either have 3, 6 or 9 arguments: + +3: apply offset and delay to read, write and flush operations on device + +6: apply offset and delay to device, also apply write_offset and write_delay + to write and flush operations on optionally different write_device with + optionally different sector offset + +9: same as 6 arguments plus define flush_offset and flush_delay explicitly + on/with optionally different flush_device/flush_offset. + +Offsets are specified in sectors. + +Delays are specified in milliseconds. + + +Example scripts +=============== + +:: + #!/bin/sh + # + # Create mapped device named "delayed" delaying read, write and flush operations for 500ms. + # + dmsetup create delayed --table "0 `blockdev --getsz $1` delay $1 0 500" + +:: + #!/bin/sh + # + # Create mapped device delaying write and flush operations for 400ms and + # splitting reads to device $1 but writes and flushes to different device $2 + # to different offsets of 2048 and 4096 sectors respectively. + # + dmsetup create delayed --table "0 `blockdev --getsz $1` delay $1 2048 0 $2 4096 400" + +:: + #!/bin/sh + # + # Create mapped device delaying reads for 50ms, writes for 100ms and flushes for 333ms + # onto the same backing device at offset 0 sectors. + # + dmsetup create delayed --table "0 `blockdev --getsz $1` delay $1 0 50 $2 0 100 $1 0 333" -- cgit v1.3.1