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([2601:188:c180:4250:ecbe:130d:668d:951d]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7e856555efbsm102176285a.11.2025.08.11.13.38.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 Aug 2025 13:38:54 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <8f357de1-72ba-497f-997a-f9a61aec6190@redhat.com> Date: Mon, 11 Aug 2025 16:38:53 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] doc: Add CPU Isolation documentation To: Frederic Weisbecker , LKML Cc: Anna-Maria Behnsen , Gabriele Monaco , Ingo Molnar , Jonathan Corbet , Marcelo Tosatti , Marco Crivellari , Michal Hocko , "Paul E . McKenney" , Peter Zijlstra , Phil Auld , Steven Rostedt , Thomas Gleixner , Valentin Schneider , Vlastimil Babka , linux-doc@vger.kernel.org References: <20250809094247.8384-1-frederic@kernel.org> Content-Language: en-US In-Reply-To: <20250809094247.8384-1-frederic@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/9/25 5:42 AM, Frederic Weisbecker wrote: > nohz_full was introduced in v3.10 in 2013, which means this > documentation is overdue for 12 years. > > The shoemaker's children always go barefoot. And working on timers > hasn't made me arriving on time either. > > 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 Thanks for working on this document. It will be really useful for those customers that need CPU isolation. > --- > Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++ > Documentation/admin-guide/index.rst | 1 + > 2 files changed, 339 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..250027acf7b2 > --- /dev/null > +++ b/Documentation/admin-guide/cpu-isolation.rst > @@ -0,0 +1,338 @@ > +============= > +CPU Isolation > +============= > + > +Introduction > +============ > + > +"CPU Isolation" means leaving a CPU exclusive to a given userspace > +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 > +significantly measurable impact most of the time. "significantly measurable" looks odd to me. Maybe "without a significant and measurable impact most of the time". > + > +Tradeoffs > +~~~~~~~~~ > + > +In terms of cost, this is the most invasive isolation feature. It is > +assumed to be used when the workload spends most of its time in > +userspace and doesn't rely on the kernel except for preparatory > +work because: > + > +- RCU is slower due to the locked, offloaded and threaded callbacks > + processing (the same that would be obtained with "rcu_nocb=" boot > + parameter). Should be "rcu_nocbs". The '=' is optional. Cheers, Longman