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/bigalloc.rst | 34 +++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) create mode 100644 Documentation/filesystems/ext4/bigalloc.rst (limited to 'Documentation/filesystems/ext4/bigalloc.rst') diff --git a/Documentation/filesystems/ext4/bigalloc.rst b/Documentation/filesystems/ext4/bigalloc.rst new file mode 100644 index 000000000..976a180b2 --- /dev/null +++ b/Documentation/filesystems/ext4/bigalloc.rst @@ -0,0 +1,34 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Bigalloc +-------- + +At the moment, the default size of a block is 4KiB, which is a commonly +supported page size on most MMU-capable hardware. This is fortunate, as +ext4 code is not prepared to handle the case where the block size +exceeds the page size. However, for a filesystem of mostly huge files, +it is desirable to be able to allocate disk blocks in units of multiple +blocks to reduce both fragmentation and metadata overhead. The +bigalloc feature provides exactly this ability. + +The bigalloc feature (EXT4_FEATURE_RO_COMPAT_BIGALLOC) changes ext4 to +use clustered allocation, so that each bit in the ext4 block allocation +bitmap addresses a power of two number of blocks. For example, if the +file system is mainly going to be storing large files in the 4-32 +megabyte range, it might make sense to set a cluster size of 1 megabyte. +This means that each bit in the block allocation bitmap now addresses +256 4k blocks. This shrinks the total size of the block allocation +bitmaps for a 2T file system from 64 megabytes to 256 kilobytes. It also +means that a block group addresses 32 gigabytes instead of 128 megabytes, +also shrinking the amount of file system overhead for metadata. + +The administrator can set a block cluster size at mkfs time (which is +stored in the s_log_cluster_size field in the superblock); from then +on, the block bitmaps track clusters, not individual blocks. This means +that block groups can be several gigabytes in size (instead of just +128MiB); however, the minimum allocation unit becomes a cluster, not a +block, even for directories. TaoBao had a patchset to extend the “use +units of clusters instead of blocks” to the extent tree, though it is +not clear where those patches went-- they eventually morphed into +“extent tree v2” but that code has not landed as of May 2015. + -- cgit v1.3.1