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McKenney" Cc: Lai Jiangshan , linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Andy Lutomirski , Fenghua Yu , Andi Kleen , Kees Cook , "Rafael J. Wysocki" , Sebastian Andrzej Siewior , Dave Hansen , Babu Moger , Rik van Riel , "Chang S. Bae" , Jann Horn , David Windsor , Elena Reshetova , Andrea Parri , Yuyang Du , Richard Guy Briggs , Anshuman Khandual , Andrew Morton , Christian Brauner , Michal Hocko , Andrea Arcangeli , Al Viro , "Dmitry V. Levin" , rcu@vger.kernel.org Subject: Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth Message-ID: <20191101131315.GY4131@hirez.programming.kicks-ass.net> References: <20191031100806.1326-1-laijs@linux.alibaba.com> <20191031100806.1326-12-laijs@linux.alibaba.com> <20191101125816.GD17910@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191101125816.GD17910@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 01, 2019 at 05:58:16AM -0700, Paul E. McKenney wrote: > On Thu, Oct 31, 2019 at 10:08:06AM +0000, Lai Jiangshan wrote: > > +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */ > > +static __always_inline int rcu_preempt_depth(void) > > +{ > > + return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL; > > Why not raw_cpu_generic_read()? > > OK, OK, I get that raw_cpu_read_4() translates directly into an "mov" > instruction on x86, but given that x86 percpu_from_op() is able to > adjust based on operand size, why doesn't something like raw_cpu_read() > also have an x86-specific definition that adjusts based on operand size? The reason for preempt.h was header recursion hell. > > +} > > + > > +static __always_inline void rcu_preempt_depth_set(int pc) > > +{ > > + int old, new; > > + > > + do { > > + old = raw_cpu_read_4(__rcu_preempt_depth); > > + new = (old & RCU_NEED_SPECIAL) | > > + (pc & ~RCU_NEED_SPECIAL); > > + } while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old); > > Ummm... > > OK, as you know, I have long wanted _rcu_read_lock() to be inlineable. > But are you -sure- that an x86 cmpxchg is faster than a function call > and return? I have strong doubts on that score. This is a regular CMPXCHG instruction, not a LOCK prefixed one, and that should make all the difference > Plus multiplying the x86-specific code by 26 doesn't look good. > > And the RCU read-side nesting depth really is a per-task thing. Copying > it to and from the task at context-switch time might make sense if we > had a serious optimization, but it does not appear that we do. > > You original patch some years back, ill-received though it was at the > time, is looking rather good by comparison. Plus it did not require > architecture-specific code! Right, so the per-cpu preempt_count code relies on the preempt_count being invariant over context switches. That means we never have to save/restore the thing. For (preemptible) rcu, this is 'obviously' not the case. That said, I've not looked over this patch series, I only got 1 actual patch, not the whole series, and I've not had time to go dig out the rest..