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Tue, 25 Jul 2023 03:10:35 -0700 (PDT) X-Google-Smtp-Source: APBJJlEBwe5grVle2BJoQfU1CdlrA5E008mXbdTKHKW2tEfZga9rph2myp9KegOF9DFTCjiBwJ8J/Q== X-Received: by 2002:a7b:c3c8:0:b0:3fc:182:7eac with SMTP id t8-20020a7bc3c8000000b003fc01827eacmr9484330wmj.33.1690279834786; Tue, 25 Jul 2023 03:10:34 -0700 (PDT) Received: from vschneid.remote.csb ([149.12.7.81]) by smtp.gmail.com with ESMTPSA id p1-20020a05600c204100b003fc17e8a1efsm15201059wmg.45.2023.07.25.03.10.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Jul 2023 03:10:34 -0700 (PDT) From: Valentin Schneider To: Frederic Weisbecker Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org, x86@kernel.org, rcu@vger.kernel.org, linux-kselftest@vger.kernel.org, Nicolas Saenz Julienne , Steven Rostedt , Masami Hiramatsu , Jonathan Corbet , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Paolo Bonzini , Wanpeng Li , Vitaly Kuznetsov , Andy Lutomirski , Peter Zijlstra , "Paul E. McKenney" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Andrew Morton , Uladzislau Rezki , Christoph Hellwig , Lorenzo Stoakes , Josh Poimboeuf , Jason Baron , Kees Cook , Sami Tolvanen , Ard Biesheuvel , Nicholas Piggin , Juerg Haefliger , Nicolas Saenz Julienne , "Kirill A. Shutemov" , Nadav Amit , Dan Carpenter , Chuang Wang , Yang Jihong , Petr Mladek , "Jason A. Donenfeld" , Song Liu , Julian Pidancet , Tom Lendacky , Dionna Glaze , Thomas =?utf-8?Q?Wei=C3=9Fschuh?= , Juri Lelli , Daniel Bristot de Oliveira , Marcelo Tosatti , Yair Podemsky Subject: Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral infrastructure In-Reply-To: References: <20230720163056.2564824-1-vschneid@redhat.com> <20230720163056.2564824-16-vschneid@redhat.com> Date: Tue, 25 Jul 2023 11:10:31 +0100 Message-ID: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Rspamd-Queue-Id: F2C58A0005 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: n7gfsgpuh7gii1hyqkyofprjy6iisi94 X-HE-Tag: 1690279838-941472 X-HE-Meta: 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 wcDWv/wy L/trGVBjIZq7xP5bDxQ2FSnW6pgCIK5yJe2reRy1jz2LaZse8GvLiDBAYRuO09k3F/BNj6ar+4WxG+j8F3gga61OYeZdDTog9XC94rhuZRuh6Y8udWO7t/awhjzF67OqWV4wyKcAsI2MTIjmV5gq44oHenVQ6CDZDrWeI7d0KcLo/mEbzTAFrLQOPyUZaKGJLAbbZsOH9ZzNA755I2mTY3drQ9w4di0dPh6QSLQKjkZMPuPNsU6SSmjLuXXqwNlQYFmAvz77SUppDWaMENlPCekrjf/Po7+MItu9VJEphfT4WZ3h5eiuAagjDbRlUaXWY+kXmkIK9AQWv38z5MskP9gbhGw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 24/07/23 21:18, Frederic Weisbecker wrote: > On Mon, Jul 24, 2023 at 05:55:44PM +0100, Valentin Schneider wrote: >> I can make that a 'do {} while ()' instead to force at least one execution >> of the cmpxchg(). >> >> This is only about reducing the race window, right? If we're executing this >> just as the target CPU is about to enter userspace, we're going to be in >> racy territory anyway. Regardless, I'm happy to do that change. > > Right, it's only about narrowing down the race window. It probably don't matter > in practice, but it's one less thing to consider for the brain :-) > Ack > Also, why bothering with handling CONTEXT_IDLE? > 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.