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, 110 insertions, 0 deletions

diff --git a/Documentation/driver-api/usb/dma.rst b/Documentation/driver-api/usb/dma.rst
new file mode 100644
index 000000000..02f6825ff
--- /dev/null
+++ b/Documentation/driver-api/usb/dma.rst
@@ -0,0 +1,110 @@
+USB DMA
+~~~~~~~
+
+In Linux 2.5 kernels (and later), USB device drivers have additional control
+over how DMA may be used to perform I/O operations.  The APIs are detailed
+in the kernel usb programming guide (kerneldoc, from the source code).
+
+API overview
+============
+
+The big picture is that USB drivers can continue to ignore most DMA issues,
+though they still must provide DMA-ready buffers (see
+Documentation/core-api/dma-api-howto.rst).  That's how they've worked through
+the 2.4 (and earlier) kernels, or they can now be DMA-aware.
+
+DMA-aware usb drivers:
+
+- New calls enable DMA-aware drivers, letting them allocate dma buffers and
+  manage dma mappings for existing dma-ready buffers (see below).
+
+- URBs have an additional "transfer_dma" field, as well as a transfer_flags
+  bit saying if it's valid.  (Control requests also have "setup_dma", but
+  drivers must not use it.)
+
+- "usbcore" will map this DMA address, if a DMA-aware driver didn't do
+  it first and set ``URB_NO_TRANSFER_DMA_MAP``.  HCDs
+  don't manage dma mappings for URBs.
+
+- There's a new "generic DMA API", parts of which are usable by USB device
+  drivers.  Never use dma_set_mask() on any USB interface or device; that
+  would potentially break all devices sharing that bus.
+
+Eliminating copies
+==================
+
+It's good to avoid making CPUs copy data needlessly.  The costs can add up,
+and effects like cache-trashing can impose subtle penalties.
+
+- If you're doing lots of small data transfers from the same buffer all
+  the time, that can really burn up resources on systems which use an
+  IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
+  tear down the IOMMU mappings with each request than perform the I/O!
+
+  For those specific cases, USB has primitives to allocate less expensive
+  memory.  They work like kmalloc and kfree versions that give you the right
+  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
+  You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags::
+
+	void *usb_alloc_coherent (struct usb_device *dev, size_t size,
+		int mem_flags, dma_addr_t *dma);
+
+	void usb_free_coherent (struct usb_device *dev, size_t size,
+		void *addr, dma_addr_t dma);
+
+  Most drivers should **NOT** be using these primitives; they don't need
+  to use this type of memory ("dma-coherent"), and memory returned from
+  :c:func:`kmalloc` will work just fine.
+
+  The memory buffer returned is "dma-coherent"; sometimes you might need to
+  force a consistent memory access ordering by using memory barriers.  It's
+  not using a streaming DMA mapping, so it's good for small transfers on
+  systems where the I/O would otherwise thrash an IOMMU mapping.  (See
+  Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and
+  "streaming" DMA mappings.)
+
+  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
+  space-efficient.
+
+  On most systems the memory returned will be uncached, because the
+  semantics of dma-coherent memory require either bypassing CPU caches
+  or using cache hardware with bus-snooping support.  While x86 hardware
+  has such bus-snooping, many other systems use software to flush cache
+  lines to prevent DMA conflicts.
+
+- Devices on some EHCI controllers could handle DMA to/from high memory.
+
+  Unfortunately, the current Linux DMA infrastructure doesn't have a sane
+  way to expose these capabilities ... and in any case, HIGHMEM is mostly a
+  design wart specific to x86_32.  So your best bet is to ensure you never
+  pass a highmem buffer into a USB driver.  That's easy; it's the default
+  behavior.  Just don't override it; e.g. with ``NETIF_F_HIGHDMA``.
+
+  This may force your callers to do some bounce buffering, copying from
+  high memory to "normal" DMA memory.  If you can come up with a good way
+  to fix this issue (for x86_32 machines with over 1 GByte of memory),
+  feel free to submit patches.
+
+Working with existing buffers
+=============================
+
+Existing buffers aren't usable for DMA without first being mapped into the
+DMA address space of the device.  However, most buffers passed to your
+driver can safely be used with such DMA mapping.  (See the first section
+of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
+
+- When you have the scatterlists which have been mapped for the USB controller,
+  you could use the new ``usb_sg_*()`` calls, which would turn scatterlist
+  into URBs::
+
+	int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev,
+		unsigned pipe, unsigned	period, struct scatterlist *sg,
+		int nents, size_t length, gfp_t mem_flags);
+
+	void usb_sg_wait(struct usb_sg_request *io);
+
+	void usb_sg_cancel(struct usb_sg_request *io);
+
+  When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try
+  to submit URBs in PIO way as long as the page in scatterlists is not in the
+  Highmem, which could be very rare in modern architectures.