From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7A3B454279; Fri, 29 Nov 2024 22:19:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732918794; cv=none; b=ElKjYT6ihijsR7xeFMNgBKlX6N+4LnxQ7V463x0N21dtiag40lu2YUTnMC0cZd0kp82lgjocDhfqlv/lpAUt7Cyt2PJ63H/FKeCCv5dEjE8xClywL1AQcpyE24NIfB4064JaYiaW8vWVPYW9M+KXRHRx5+JOXdjy94Zk/iCWLYc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732918794; c=relaxed/simple; bh=OJvU5bLoYGZXh8qACQ/T6IEV8oDjNatqYL+CeW3HsJg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=i6J+y4trG6gaP981SbMfKlABk0A+efDzgh2pSzsX4f2H5OVWBI+A+eLx/O0G9lnsKR0kdOK4WeFSIDRs5Iy7DE32Hy3TwL9zQxTf6mmSqqtklFQRdGDL7weOpHUjJzfdf5YN9hI+7nEYJ7PLJV4SkVAOHiijyaZUHriuaHgpkrk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=o5p0Hl5t; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="o5p0Hl5t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8C894C4CECF; Fri, 29 Nov 2024 22:19:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1732918794; bh=OJvU5bLoYGZXh8qACQ/T6IEV8oDjNatqYL+CeW3HsJg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=o5p0Hl5t68sOGrNrYENKMovE5NztdhOpEAFABibzPZGUoqz1AXXaCH53QUmqJu9yj 4Ymv5lTSZbEsDt+NPkbGOI0BMOSI/hhFnrmJFW+/qqVN7darYuDK67nomYfyn1WTZW BFRJMR/Fl9FeauFwTeTH3phw520rl/KLfJopadlpHQUp+IZrgAicwA5DrSA19U8kSy DpTFETeRTKi33P/1gcVkoaLTCtVby2dZtM0i0fCZSI8+UVXXGJuKWkaLG6dG9bq/8l w9ueF6Q3rudD7ZWIlpxFt0GwISNHkjcxLh8drdyWqw8+a9ohium7+Z6XshqrmZWZxx DWCgNchAqIpvQ== Date: Fri, 29 Nov 2024 23:19:51 +0100 From: Frederic Weisbecker To: Valentin Schneider Cc: linux-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 =?iso-8859-1?Q?Wei=DFschuh?= , Juri Lelli , Marcelo Tosatti , Yair Podemsky , Daniel Wagner , Petr Tesarik Subject: Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral infrastructure Message-ID: References: <20241119153502.41361-1-vschneid@redhat.com> <20241119153502.41361-12-vschneid@redhat.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Le Fri, Nov 29, 2024 at 05:40:29PM +0100, Valentin Schneider a écrit : > On 24/11/24 22:46, Frederic Weisbecker wrote: > > Le Fri, Nov 22, 2024 at 03:56:59PM +0100, Valentin Schneider a écrit : > >> On 20/11/24 18:30, Frederic Weisbecker wrote: > >> > Le Wed, Nov 20, 2024 at 06:10:43PM +0100, Valentin Schneider a écrit : > >> >> On 20/11/24 15:23, Frederic Weisbecker wrote: > >> >> > >> >> > Ah but there is CT_STATE_GUEST and I see the last patch also applies that to > >> >> > CT_STATE_IDLE. > >> >> > > >> >> > So that could be: > >> >> > > >> >> > bool ct_set_cpu_work(unsigned int cpu, unsigned int work) > >> >> > { > >> >> > struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu); > >> >> > unsigned int old; > >> >> > bool ret = false; > >> >> > > >> >> > preempt_disable(); > >> >> > > >> >> > old = atomic_read(&ct->state); > >> >> > > >> >> > /* CT_STATE_IDLE can be added to last patch here */ > >> >> > if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) { > >> >> > old &= ~CT_STATE_MASK; > >> >> > old |= CT_STATE_USER; > >> >> > } > >> >> > >> >> Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check, > >> >> but we get an extra loop if the target CPU exits kernelspace not to > >> >> userspace (e.g. vcpu or idle) in the meantime - not great, not terrible. > >> > > >> > The thing is, what you read with atomic_read() should be close to reality. > >> > If it already is != CT_STATE_KERNEL then you're good (minus racy changes). > >> > If it is CT_STATE_KERNEL then you still must do a failing cmpxchg() in any case, > >> > at least to make sure you didn't miss a context tracking change. So the best > >> > you can do is a bet. > >> > > >> >> > >> >> At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1 > >> >> we could do: > >> >> > >> >> old = atomic_read(&ct->state); > >> >> old &= ~CT_STATE_KERNEL; > >> > > >> > And perhaps also old |= CT_STATE_IDLE (I'm seeing the last patch now), > >> > so you at least get a chance of making it right (only ~CT_STATE_KERNEL > >> > will always fail) and CPUs usually spend most of their time idle. > >> > > >> > >> I'm thinking with: > >> > >> CT_STATE_IDLE = 0, > >> CT_STATE_USER = 1, > >> CT_STATE_GUEST = 2, > >> CT_STATE_KERNEL = 4, /* Keep that as a standalone bit */ > > > > Right! > > > >> > >> we can stick with old &= ~CT_STATE_KERNEL; and that'll let the cmpxchg > >> succeed for any of IDLE/USER/GUEST. > > > > Sure but if (old & CT_STATE_KERNEL), cmpxchg() will consistently fail. > > But you can make a bet that it has switched to CT_STATE_IDLE between > > the atomic_read() and the first atomic_cmpxchg(). This way you still have > > a tiny chance to succeed. > > > > That is: > > > > old = atomic_read(&ct->state); > > if (old & CT_STATE_KERNEl) > > old |= CT_STATE_IDLE; > > old &= ~CT_STATE_KERNEL; > > > > > > do { > > atomic_try_cmpxchg(...) > > > > Hmm? > > But it could equally be CT_STATE_{USER, GUEST}, right? That is, if we have > all of this enabled them we assume the isolated CPUs spend the least amount > of time in the kernel, if they don't we get to blame the user. Unless CONTEXT_TRACKING_WORK_IDLE=y yes. Anyway that's just a detail that can be refined in the future. I'm fine with just clearing CT_STATE_KERNEL and go with that. Thanks.