Messages in this thread | | | Subject | Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth | From | Lai Jiangshan <> | Date | Fri, 1 Nov 2019 23:47:18 +0800 |
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On 2019/11/1 9:13 下午, Peter Zijlstra wrote: > 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.
Oh, I didn't notice. May we can use raw_cpu_generic_read for rcu here, I will have a try.
Thanks Lai.
> >>> +} >>> + >>> +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.. >
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