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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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 10B0B140026 X-Stat-Signature: p5tw6ceexazknedqru53qh9yb46ne3dt X-Rspam-User: X-HE-Tag: 1732123787-456588 X-HE-Meta: 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 aRUJ+JLv uf+Mr5JLEMMn4L9g2dVL3k4RJC0Mbby/HvRQ1pOFzxipAv+mIGBnnmMOaaNoyrsRhWTSRhjgT75pR2K/G8bFkm3c+tFSyfTZs2g8JL7oWpacatTpbUngR3GndJEtsgWiV9W2l2Wd0meriF6sasQCC94gRKaUKoZ/Tyg8aHV/10G3kjgTZKMg8ky0h02CU3Bh9RtvWqqm783/RVMQTlBYMIwQT0Q7LJyuHg+8LSO2soW5SMCVYwdBqErixFyv2yD+X5sMBNRHzZ05p6NZzAlVLucmyK5WLJtMxCJyvIsuk/iF5gL8ileKoJktMXQ== 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: List-Subscribe: List-Unsubscribe: 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. Thanks.