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 43F112500BB; Sun, 24 Nov 2024 21:46:04 +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=1732484764; cv=none; b=kYM5bQmZSVHSvWuWczJfOe8dMwF44gWf0nTxk8htqeliEQsZ1hcQyT/hCsldQh9O85i0fraFcex97vJ6iawOMiGEhxzC5g6KHqU+f1OYRitWdCyu4v9DDxPxVDeFst7XF54Jh1uDD742RAOFvrJb9o0FqvIWRkWMk5hCuQwLfXY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732484764; c=relaxed/simple; bh=/qhmFmtGTN/yG2NensqYHITpZ/haNfld1Woay8hK220=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nVpy1P9Z6baVEDcUdk8nWBa/qcNsdWfLH92XwRiQ5WKSSLHpFqlfrMnT1Nl3v/FvklT9OHJPp29+yAotpBYrJaCq0Z9sKJWsWQZMUC+fjkPEs3E3U9hTWIRWnAKOa0eAKZ0PFTonQ8tlXZtheAJDWz8t0l7FuRpWRnISz5Ovvh0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZNg7MJ2I; 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="ZNg7MJ2I" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5EE06C4CECC; Sun, 24 Nov 2024 21:46:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1732484763; bh=/qhmFmtGTN/yG2NensqYHITpZ/haNfld1Woay8hK220=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ZNg7MJ2IIU3CeTaHZ48HzrmcZZAw0SYc+qCR/IKYfojob4+a9LbntBCi696q0OjUN 3nQvd8ihsO2okerTSi3qXbeP7+LNX7rk2RZTot29gYFw3eku4E/jStldwicIm8jQSq Jpvge98weBKXSdbt5FHI9opC+dsG48zcJQqC4EpSMqFwt5BAyZtTlcIZ8F1rOycM8t gBXJmsYnFDfxgSh+rxCGS5FXmXw2NvRkPX7hznrkisrgC//vxx85ldSGSXAXmR7YTU 3LHIuIqDGcPS9H2ByV200QW0+2+eVwzg+CINe8g2Q9xl4+OtQ3aiK5W+MZpdqpHY8C k9ocOhptHnk2g== Date: Sun, 24 Nov 2024 22:46:01 +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 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?