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Feng Tang b4bac27931 x86/tsc: Use topology_max_packages() to get package number 
Commit b50db7095f ("x86/tsc: Disable clocksource watchdog for TSC on
qualified platorms") was introduced to solve problem that sometimes TSC
clocksource is wrongly judged as unstable by watchdog like 'jiffies', HPET,
etc.

In it, the hardware package number is a key factor for judging whether to
disable the watchdog for TSC, and 'nr_online_nodes' was chosen due to, at
that time (kernel v5.1x), it is available in early boot phase before
registering 'tsc-early' clocksource, where all non-boot CPUs are not
brought up yet.

Dave and Rui pointed out there are many cases in which 'nr_online_nodes'
is cheated and not accurate, like:

 * SNC (sub-numa cluster) mode enabled
 * numa emulation (numa=fake=8 etc.)
 * numa=off
 * platforms with CPU-less HBM nodes, CPU-less Optane memory nodes.
 * 'maxcpus=' cmdline setup, where chopped CPUs could be onlined later
 * 'nr_cpus=', 'possible_cpus=' cmdline setup, where chopped CPUs can
   not be onlined after boot

The SNC case is the most user-visible case, as many CSP (Cloud Service
Provider) enable this feature in their server fleets. When SNC3 enabled, a
2 socket machine will appear to have 6 NUMA nodes, and get impacted by the
issue in reality.

Thomas' recent patchset of refactoring x86 topology code improves
topology_max_packages() greatly, by making it more accurate and available
in early boot phase, which works well in most of the above cases.

The only exceptions are 'nr_cpus=' and 'possible_cpus=' setup, which may
under-estimate the package number. As during topology setup, the boot CPU
iterates through all enumerated APIC IDs and either accepts or rejects the
APIC ID. For accepted IDs, it figures out which bits of the ID map to the
package number.  It tracks which package numbers have been seen in a
bitmap.  topology_max_packages() just returns the number of bits set in
that bitmap.

'nr_cpus=' and 'possible_cpus=' can cause more APIC IDs to be rejected and
can artificially lower the number of bits in the package bitmap and thus
topology_max_packages().  This means that, for example, a system with 8
physical packages might reject all the CPUs on 6 of those packages and be
left with only 2 packages and 2 bits set in the package bitmap. It needs
the TSC watchdog, but would disable it anyway.  This isn't ideal, but it
only happens for debug-oriented options. This is fixable by tracking the
package numbers for rejected CPUs.  But it's not worth the trouble for
debugging.

So use topology_max_packages() to replace nr_online_nodes().

Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/all/20240729021202.180955-1-feng.tang@intel.com
Closes: https://lore.kernel.org/lkml/a4860054-0f16-6513-f121-501048431086@intel.com/
History 2024-07-31 21:12:09 +02:00
arch x86/tsc: Use topology_max_packages() to get package number 2024-07-31 21:12:09 +02:00
block block: fix deadlock between sd_remove & sd_release 2024-07-24 09:51:21 -06:00
certs kbuild: use $(src) instead of $(srctree)/$(src) for source directory 2024-05-10 04:34:52 +09:00
crypto crypto: testmgr - generate power-of-2 lengths more often 2024-07-13 11:50:28 +12:00
Documentation CXL for v6.11 merge window 2024-07-28 09:33:28 -07:00
drivers minmax: add a few more MIN_T/MAX_T users 2024-07-28 13:41:14 -07:00
fs This pull request contains updates (actually, just fixes) for UBI and UBIFS: 2024-07-28 11:51:51 -07:00
include minmax: simplify and clarify min_t()/max_t() implementation 2024-07-28 13:50:01 -07:00
init Rust changes for v6.11 2024-07-27 13:44:54 -07:00
io_uring io_uring/napi: pass ktime to io_napi_adjust_timeout 2024-07-26 08:31:59 -06:00
ipc sysctl: treewide: constify the ctl_table argument of proc_handlers 2024-07-24 20:59:29 +02:00
kernel Fixes and minor updates for the timer migration code: 2024-07-27 10:19:55 -07:00
lib Rust changes for v6.11 2024-07-27 13:44:54 -07:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm mm/page_alloc: fix pcp->count race between drain_pages_zone() vs __rmqueue_pcplist() 2024-07-26 14:33:09 -07:00
net minmax: add a few more MIN_T/MAX_T users 2024-07-28 13:41:14 -07:00
rust Rust changes for v6.11 2024-07-27 13:44:54 -07:00
samples Driver core changes for 6.11-rc1 2024-07-25 10:42:22 -07:00
scripts Kbuild fixes for v6.11 2024-07-28 14:02:48 -07:00
security apparmor-pr-2024-07-24 PR 2024-07-25 2024-07-27 13:28:39 -07:00
sound Devicetree fixes for 6.11, part 1 2024-07-27 12:46:16 -07:00
tools turbostat release 2024.07.26 2024-07-28 10:52:15 -07:00
usr initramfs: shorten cmd_initfs in usr/Makefile 2024-07-16 01:07:52 +09:00
virt KVM generic changes for 6.11 2024-07-16 09:51:36 -04:00
.clang-format Docs: Move clang-format from process/ to dev-tools/ 2024-06-26 16:36:00 -06:00
.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 Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: add script and target to generate pacman package 2024-07-22 01:24:22 +09:00
.mailmap MAINTAINERS: mailmap: update James Clark's email address 2024-07-26 14:32:35 -07:00
.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 tracing: Update of MAINTAINERS and CREDITS file 2024-07-18 14:08:42 -07:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS CXL for v6.11 merge window 2024-07-28 09:33:28 -07:00
Makefile Linux 6.11-rc1 2024-07-28 14:19:55 -07:00
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.