Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

[KOSAKI Motohiro]

> * KOSAKI Motohiro (kosaki.motohiro@xxxxxxxxxxxxxx) wrote:
> > > * KOSAKI Motohiro (kosaki.motohiro@xxxxxxxxxxxxxx) wrote:
> [...]
> > > > It depend on what mean "constant overhead". kmalloc might cause
> > > > page reclaim and undeterministic delay. I'm not sure (1) How much
> > > > membarrier_retry() slower than smp_call_function_many and (2) Which do
> > > > you think important average or worst performance. Only I note I don't
> > > > think GFP_KERNEL is constant overhead.
> > > 
> > > 10,000,000 sys_membarrier calls (varying the number of threads to which
> > > we send IPIs), IPI-to-many, 8-core system:
> > > 
> > > T=1: 0m20.173s
> > > T=2: 0m20.506s
> > > T=3: 0m22.632s
> > > T=4: 0m24.759s
> > > T=5: 0m26.633s
> > > T=6: 0m29.654s
> > > T=7: 0m30.669s
> > > 
> > > Just doing local mb()+single IPI to T other threads:
> > > 
> > > T=1: 0m18.801s
> > > T=2: 0m29.086s
> > > T=3: 0m46.841s
> > > T=4: 0m53.758s
> > > T=5: 1m10.856s
> > > T=6: 1m21.142s
> > > T=7: 1m38.362s
> > > 
> > > So sending single IPIs adds about 1.5 microseconds per extra core. With
> > > the IPI-to-many scheme, we add about 0.2 microseconds per extra core. So
> > > we have a factor 10 gain in scalability. The initial cost of the cpumask
> > > allocation (which seems to be allocated on the stack in my config) is
> > > just about 1.4 microseconds. So here, we only have a small gain for the
> > > 1 IPI case, which does not justify the added complexity of dealing with
> > > it differently.
> > 
> > I'd like to discuss to separate CONFIG_CPUMASK_OFFSTACK=1 and CONFIG_CPUMASK_OFFSTACK=0.
> > 
> > CONFIG_CPUMASK_OFFSTACK=0 (your config)
> > 	- cpumask is allocated on stask
> > 	- alloc_cpumask_var() is nop (yes, nop is constant overhead ;)
> > 	- alloc_cpumask_var() always return 1, then membarrier_retry() is never called.
> > 	- alloc_cpumask_var() ignore GFP_KERNEL parameter
> > 
> > CONFIG_CPUMASK_OFFSTACK=1 and use GFP_KERNEL
> > 	- cpumask is allocated on heap
> > 	- alloc_cpumask_var() is the wrapper of kmalloc()
> > 	- GFP_KERNEL parameter is passed kmalloc
> > 	- GFP_KERNEL mean alloc_cpumask_var() always return 1, except
> > 	  oom-killer case. IOW, membarrier_retry() is still never called
> > 	  on typical use case.
> > 	- kmalloc(GFP_KERNEL) might invoke page reclaim and it can spent few
> > 	  seconds (not microseconds).
> > 
> > CONFIG_CPUMASK_OFFSTACK=1 and use GFP_ATOMIC
> > 	- cpumask is allocated on heap
> > 	- alloc_cpumask_var() is the wrapper of kmalloc()
> > 	- GFP_ATOMIC mean kmalloc never invoke page reclaim. IOW, 
> > 	  kmalloc() cost is nearly constant. (few or lots microseconds)
> > 	- OTOH, alloc_cpumask_var() might fail, at that time membarrier_retry()
> > 	  is called.
> > 
> > So, My last mail talked about CONFIG_CPUMASK_OFFSTACK=1, but you mesured CONFIG_CPUMASK_OFFSTACK=0.
> > That's the reason why our conclusion is different.
> 
> I would have to put my system in OOM condition anyway to measure the
> page reclaim overhead. Given that sys_membarrier is not exactly a fast
> path, I don't think it matters _that much_.
> 
> Hrm. Well, given the "expedited" nature of the system call, it might
> come as a surprise to have to wait for page reclaim, and surprises are
> not good. OTOH, I don't want to allow users to easily consume all the
> GFP_ATOMIC pool. But I think it's unlikely, as we are bounded by the
> number of processors which can concurrently run sys_membarrier().

GFP_NOWAIT prevent such consuming GFP_ATOMIC pool. but yes, you already
answered i wanted, "sys_membarrier is not exactly a fast path, I don't
think it matters _that much_.".
okay, i understand librcu latency policy. iow, i agree your patch.

Thanks lots explanation.


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