linux-kernel - Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral infrastructure

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 25 Jul 2023 14:05:47 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org, x86@...nel.org, rcu@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nadav Amit <namit@...are.com>,
        Dan Carpenter <error27@...il.com>,
        Chuang Wang <nashuiliang@...il.com>,
        Yang Jihong <yangjihong1@...wei.com>,
        Petr Mladek <pmladek@...e.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>, Song Liu <song@...nel.org>,
        Julian Pidancet <julian.pidancet@...cle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Yair Podemsky <ypodemsk@...hat.com>
Subject: Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral
 infrastructure

On 25/07/23 13:22, Frederic Weisbecker wrote:
> On Tue, Jul 25, 2023 at 11:10:31AM +0100, Valentin Schneider wrote:
>> I have reasons! I just swept them under the rug and didn't mention them :D
>> Also looking at the config dependencies again I got it wrong, but
>> nevertheless that means I get to ramble about it.
>>
>> With NO_HZ_IDLE, we get CONTEXT_TRACKING_IDLE, so we get these
>> transitions:
>>
>>   ct_idle_enter()
>>     ct_kernel_exit()
>>       ct_state_inc_clear_work()
>>
>>   ct_idle_exit()
>>     ct_kernel_enter()
>>       ct_work_flush()
>>
>> Now, if we just make CONTEXT_TRACKING_WORK depend on CONTEXT_TRACKING_IDLE
>> rather than CONTEXT_TRACKING_USER, we get to leverage the IPI deferral for
>> NO_HZ_IDLE kernels - in other words, we get to keep idle CPUs idle longer.
>>
>> It's a completely different argument than reducing interference for
>> NOHZ_FULL userspace applications and I should have at the very least
>> mentioned it in the cover letter, but it's the exact same backing
>> mechanism.
>>
>> Looking at it again, I'll probably make the CONTEXT_IDLE thing a separate
>> patch with a proper changelog.
>
> Ok should that be a seperate Kconfig? This indeed can bring power improvement
> but at the cost of more overhead from the sender. A balance to be measured...

Yep agreed, I'll make that an optional config.