sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

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 <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 127 insertions, 0 deletions

diff --git a/block/partitions/sun.c b/block/partitions/sun.c
new file mode 100644
index 000000000..92c645fcd
--- /dev/null
+++ b/block/partitions/sun.c
@@ -0,0 +1,127 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  fs/partitions/sun.c
+ *
+ *  Code extracted from drivers/block/genhd.c
+ *
+ *  Copyright (C) 1991-1998  Linus Torvalds
+ *  Re-organised Feb 1998 Russell King
+ */
+
+#include "check.h"
+
+#define SUN_LABEL_MAGIC          0xDABE
+#define SUN_VTOC_SANITY          0x600DDEEE
+
+enum {
+	SUN_WHOLE_DISK = 5,
+	LINUX_RAID_PARTITION = 0xfd,	/* autodetect RAID partition */
+};
+
+int sun_partition(struct parsed_partitions *state)
+{
+	int i;
+	__be16 csum;
+	int slot = 1;
+	__be16 *ush;
+	Sector sect;
+	struct sun_disklabel {
+		unsigned char info[128];   /* Informative text string */
+		struct sun_vtoc {
+		    __be32 version;     /* Layout version */
+		    char   volume[8];   /* Volume name */
+		    __be16 nparts;      /* Number of partitions */
+		    struct sun_info {           /* Partition hdrs, sec 2 */
+			__be16 id;
+			__be16 flags;
+		    } infos[8];
+		    __be16 padding;     /* Alignment padding */
+		    __be32 bootinfo[3];  /* Info needed by mboot */
+		    __be32 sanity;       /* To verify vtoc sanity */
+		    __be32 reserved[10]; /* Free space */
+		    __be32 timestamp[8]; /* Partition timestamp */
+		} vtoc;
+		__be32 write_reinstruct; /* sectors to skip, writes */
+		__be32 read_reinstruct;  /* sectors to skip, reads */
+		unsigned char spare[148]; /* Padding */
+		__be16 rspeed;     /* Disk rotational speed */
+		__be16 pcylcount;  /* Physical cylinder count */
+		__be16 sparecyl;   /* extra sects per cylinder */
+		__be16 obs1;       /* gap1 */
+		__be16 obs2;       /* gap2 */
+		__be16 ilfact;     /* Interleave factor */
+		__be16 ncyl;       /* Data cylinder count */
+		__be16 nacyl;      /* Alt. cylinder count */
+		__be16 ntrks;      /* Tracks per cylinder */
+		__be16 nsect;      /* Sectors per track */
+		__be16 obs3;       /* bhead - Label head offset */
+		__be16 obs4;       /* ppart - Physical Partition */
+		struct sun_partition {
+			__be32 start_cylinder;
+			__be32 num_sectors;
+		} partitions[8];
+		__be16 magic;      /* Magic number */
+		__be16 csum;       /* Label xor'd checksum */
+	} * label;
+	struct sun_partition *p;
+	unsigned long spc;
+	int use_vtoc;
+	int nparts;
+
+	label = read_part_sector(state, 0, &sect);
+	if (!label)
+		return -1;
+
+	p = label->partitions;
+	if (be16_to_cpu(label->magic) != SUN_LABEL_MAGIC) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	/* Look at the checksum */
+	ush = ((__be16 *) (label+1)) - 1;
+	for (csum = 0; ush >= ((__be16 *) label);)
+		csum ^= *ush--;
+	if (csum) {
+		printk("Dev %s Sun disklabel: Csum bad, label corrupted\n",
+		       state->disk->disk_name);
+		put_dev_sector(sect);
+		return 0;
+	}
+
+	/* Check to see if we can use the VTOC table */
+	use_vtoc = ((be32_to_cpu(label->vtoc.sanity) == SUN_VTOC_SANITY) &&
+		    (be32_to_cpu(label->vtoc.version) == 1) &&
+		    (be16_to_cpu(label->vtoc.nparts) <= 8));
+
+	/* Use 8 partition entries if not specified in validated VTOC */
+	nparts = (use_vtoc) ? be16_to_cpu(label->vtoc.nparts) : 8;
+
+	/*
+	 * So that old Linux-Sun partitions continue to work,
+	 * alow the VTOC to be used under the additional condition ...
+	 */
+	use_vtoc = use_vtoc || !(label->vtoc.sanity ||
+				 label->vtoc.version || label->vtoc.nparts);
+	spc = be16_to_cpu(label->ntrks) * be16_to_cpu(label->nsect);
+	for (i = 0; i < nparts; i++, p++) {
+		unsigned long st_sector;
+		unsigned int num_sectors;
+
+		st_sector = be32_to_cpu(p->start_cylinder) * spc;
+		num_sectors = be32_to_cpu(p->num_sectors);
+		if (num_sectors) {
+			put_partition(state, slot, st_sector, num_sectors);
+			state->parts[slot].flags = 0;
+			if (use_vtoc) {
+				if (be16_to_cpu(label->vtoc.infos[i].id) == LINUX_RAID_PARTITION)
+					state->parts[slot].flags |= ADDPART_FLAG_RAID;
+				else if (be16_to_cpu(label->vtoc.infos[i].id) == SUN_WHOLE_DISK)
+					state->parts[slot].flags |= ADDPART_FLAG_WHOLEDISK;
+			}
+		}
+		slot++;
+	}
+	seq_buf_puts(&state->pp_buf, "\n");
+	put_dev_sector(sect);
+	return 1;
+}