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McKenney" , Peter Zijlstra , Phil Auld , Steven Rostedt , Thomas Gleixner , Valentin Schneider , Vlastimil Babka , linux-doc@vger.kernel.org, Sebastian Andrzej Siewior , Bagas Sanjaya References: <20260326140055.41555-1-frederic@kernel.org> <90a6512f-5b6b-4781-87f3-4580ac426c37@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On 4/1/26 11:47 AM, Frederic Weisbecker wrote: > Le Thu, Mar 26, 2026 at 03:17:48PM -0400, Waiman Long a écrit : >> On 3/26/26 10:00 AM, Frederic Weisbecker wrote: >>> nohz_full was introduced in v3.10 in 2013, which means this >>> documentation is overdue for 13 years. >>> >>> Fortunately Paul wrote a part of the needed documentation a while ago, >>> especially concerning nohz_full in Documentation/timers/no_hz.rst and >>> also about per-CPU kthreads in >>> Documentation/admin-guide/kernel-per-CPU-kthreads.rst >>> >>> Introduce a new page that gives an overview of CPU isolation in general. >>> >>> Signed-off-by: Frederic Weisbecker >>> --- >>> v2: >>> - Fix links and code blocks (Bagas and Sebastian) >>> - Isolation is not only about userspace, rephrase accordingly (Valentin) >>> - Paste BIOS issues suggestion from Valentin >>> - Include the whole rtla suite (Valentin) >>> - Rephrase a few details (Waiman) >>> - Talk about RCU induced overhead rather than slower RCU (Sebastian) >>> >>> Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++ >>> Documentation/admin-guide/index.rst | 1 + >>> 2 files changed, 358 insertions(+) >>> create mode 100644 Documentation/admin-guide/cpu-isolation.rst >>> >>> diff --git a/Documentation/admin-guide/cpu-isolation.rst b/Documentation/admin-guide/cpu-isolation.rst >>> new file mode 100644 >>> index 000000000000..886dec79b056 >>> --- /dev/null >>> +++ b/Documentation/admin-guide/cpu-isolation.rst >>> @@ -0,0 +1,357 @@ >>> +.. SPDX-License-Identifier: GPL-2.0 >>> + >>> +============= >>> +CPU Isolation >>> +============= >>> + >>> +Introduction >>> +============ >>> + >>> +"CPU Isolation" means leaving a CPU exclusive to a given workload >>> +without any undesired code interference from the kernel. >>> + >>> +Those interferences, commonly pointed out as "noise", can be triggered >>> +by asynchronous events (interrupts, timers, scheduler preemption by >>> +workqueues and kthreads, ...) or synchronous events (syscalls and page >>> +faults). >>> + >>> +Such noise usually goes unnoticed. After all synchronous events are a >>> +component of the requested kernel service. And asynchronous events are >>> +either sufficiently well distributed by the scheduler when executed >>> +as tasks or reasonably fast when executed as interrupt. The timer >>> +interrupt can even execute 1024 times per seconds without a significant >>> +and measurable impact most of the time. >>> + >>> +However some rare and extreme workloads can be quite sensitive to >>> +those kinds of noise. This is the case, for example, with high >>> +bandwidth network processing that can't afford losing a single packet >>> +or very low latency network processing. Typically those usecases >>> +involve DPDK, bypassing the kernel networking stack and performing >>> +direct access to the networking device from userscace. >> As also pointed by by Sashiko, there is a typo "userscace" -> "userspace". >> There are also typos reported in >> >> https://sashiko.dev/#/patchset/20260326140055.41555-1-frederic%40kernel.org > Thanks! > > What do you think about these lines of Sashiko's review: > > """ > Does this script violate the cgroup v2 "no internal process" constraint? > By enabling the cpuset controller on the test directory's > cgroup.subtree_control file, the cgroup cannot also contain processes. > """ > > That is confusing me... I would say that the "no internal process" is a suggestion for the cgroup setup. The real world is actually more complicated. So I will just ignore that. Cheers, Longman