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Peter Zijlstra b29d5e3a56 sched/eevdf: Fix min_vruntime vs avg_vruntime 
[ Upstream commit 79f3f9bedd ]

Basically, from the constraint that the sum of lag is zero, you can
infer that the 0-lag point is the weighted average of the individual
vruntime, which is what we're trying to compute:

        \Sum w_i * v_i
  avg = --------------
           \Sum w_i

Now, since vruntime takes the whole u64 (worse, it wraps), this
multiplication term in the numerator is not something we can compute;
instead we do the min_vruntime (v0 henceforth) thing like:

  v_i = (v_i - v0) + v0

This does two things:
 - it keeps the key: (v_i - v0) 'small';
 - it creates a relative 0-point in the modular space.

If you do that subtitution and work it all out, you end up with:

        \Sum w_i * (v_i - v0)
  avg = --------------------- + v0
              \Sum w_i

Since you cannot very well track a ratio like that (and not suffer
terrible numerical problems) we simpy track the numerator and
denominator individually and only perform the division when strictly
needed.

Notably, the numerator lives in cfs_rq->avg_vruntime and the denominator
lives in cfs_rq->avg_load.

The one extra 'funny' is that these numbers track the entities in the
tree, and current is typically outside of the tree, so avg_vruntime()
adds current when needed before doing the division.

(vruntime_eligible() elides the division by cross-wise multiplication)

Anyway, as mentioned above, we currently use the CFS era min_vruntime
for this purpose. However, this thing can only move forward, while the
above avg can in fact move backward (when a non-eligible task leaves,
the average becomes smaller), this can cause trouble when through
happenstance (or construction) these values drift far enough apart to
wreck the game.

Replace cfs_rq::min_vruntime with cfs_rq::zero_vruntime which is kept
near/at avg_vruntime, following its motion.

The down-side is that this requires computing the avg more often.

Fixes: 147f3efaa2 ("sched/fair: Implement an EEVDF-like scheduling policy")
Reported-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251106111741.GC4068168@noisy.programming.kicks-ass.net
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-01-08 10:16:41 +01:00
arch x86/microcode/AMD: Select which microcode patch to load 2026-01-08 10:16:41 +01:00
block block: freeze queue when updating zone resources 2026-01-02 12:57:31 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: af_alg - zero initialize memory allocated via sock_kmalloc 2026-01-02 12:57:27 +01:00
Documentation dt-bindings: PCI: qcom,pcie-sm8550: Add missing required power-domains and resets 2026-01-02 12:57:27 +01:00
drivers drm: nova: depend on CONFIG_64BIT 2026-01-08 10:16:41 +01:00
fs fuse: missing copy_finish in fuse-over-io-uring argument copies 2026-01-02 12:57:31 +01:00
include mptcp: pm: ignore unknown endpoint flags 2026-01-02 12:57:12 +01:00
init printk changes for 6.18 2025-10-04 11:13:11 -07:00
io_uring io_uring/rsrc: fix lost entries after cloned range 2026-01-02 12:57:30 +01:00
ipc namespace-6.18-rc1 2025-09-29 11:20:29 -07:00
kernel sched/eevdf: Fix min_vruntime vs avg_vruntime 2026-01-08 10:16:41 +01:00
lib lib/crypto: riscv: Depend on RISCV_EFFICIENT_VECTOR_UNALIGNED_ACCESS 2026-01-02 12:57:29 +01:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/slub: reset KASAN tag in defer_free() before accessing freed memory 2026-01-02 12:57:18 +01:00
net SUNRPC: svcauth_gss: avoid NULL deref on zero length gss_token in gss_read_proxy_verf 2026-01-02 12:57:25 +01:00
rust rust: io: add typedef for phys_addr_t 2026-01-02 12:57:05 +01:00
samples samples: rust: fix endianness issue in rust_driver_pci 2026-01-02 12:57:05 +01:00
scripts kbuild: Use objtree for module signing key path 2026-01-02 12:56:40 +01:00
security tpm2-sessions: Fix out of range indexing in name_size 2026-01-02 12:57:11 +01:00
sound ASoC: SOF: ipc4-topology: Convert FLOAT to S32 during blob selection 2026-01-02 12:57:24 +01:00
tools rtla/timerlat_bpf: Stop tracing on user latency 2026-01-02 12:57:29 +01:00
usr gen_init_cpio: Ignore fsync() returning EINVAL on pipes 2025-10-07 09:53:05 -07:00
virt KVM: Fix last_boosted_vcpu index assignment bug 2026-01-02 12:57:21 +01:00
.clang-format memblock: drop for_each_free_mem_pfn_range_in_zone_from() 2025-09-14 08:49:03 +03:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore .gitignore: ignore compile_commands.json globally 2025-08-12 15:53:55 -07:00
.mailmap 8 hotfixes. 4 are cc:stable, 7 are against mm/. 2025-11-26 12:38:05 -08:00
.pylintrc tools: docs: parse-headers.py: move it from sphinx dir 2025-08-29 15:54:42 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: mark ISDN subsystem as orphan 2025-10-27 17:49:45 -07:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS soc: fixes for 6.18, part 4 2025-11-28 09:57:31 -08:00
Makefile Linux 6.18.3 2026-01-02 12:57:31 +01:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.