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/filesystems/ext4/bitmaps.rst | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) create mode 100644 Documentation/filesystems/ext4/bitmaps.rst (limited to 'Documentation/filesystems/ext4/bitmaps.rst') diff --git a/Documentation/filesystems/ext4/bitmaps.rst b/Documentation/filesystems/ext4/bitmaps.rst new file mode 100644 index 000000000..9d7d7b083 --- /dev/null +++ b/Documentation/filesystems/ext4/bitmaps.rst @@ -0,0 +1,21 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Block and inode Bitmaps +----------------------- + +The data block bitmap tracks the usage of data blocks within the block +group. + +The inode bitmap records which entries in the inode table are in use. + +As with most bitmaps, one bit represents the usage status of one data +block or inode table entry. This implies a block group size of 8 * +number_of_bytes_in_a_logical_block. + +NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts +of the kernel and e2fsprogs code pretends that the block bitmap contains +zeros (i.e. all blocks in the group are free). However, it is not +necessarily the case that no blocks are in use -- if ``meta_bg`` is set, +the bitmaps and group descriptor live inside the group. Unfortunately, +ext2fs_test_block_bitmap2() will return '0' for those locations, +which produces confusing debugfs output. -- cgit v1.3.1