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Date:   Fri, 31 Aug 2018 10:04:42 +0100
From:   Patrick Bellasi <patrick.bellasi@arm.com>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     peterz@infradead.org, rjw@rjwysocki.net,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
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        javi.merino@kernel.org
Subject: Re: [PATCH v6 03/14] PM: Introduce an Energy Model management
 framework
Message-ID: <20180831090442.GA25636@e110439-lin>
References: <20180820094420.26590-1-quentin.perret@arm.com>
 <20180820094420.26590-4-quentin.perret@arm.com>
 <20180829100435.GP2960@e110439-lin>
 <20180829132811.iacfltcos6kfgp7e@queper01-lin>
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On 29-Aug 14:28, Quentin Perret wrote:
> Hi Patrick,
> 
> On Wednesday 29 Aug 2018 at 11:04:35 (+0100), Patrick Bellasi wrote:
> > In the loop above we use smp_store_release() to propagate the pointer
> > setting in a PER_CPU(em_data), which ultimate goal is to protect
> > em_register_perf_domain() from multiple clients registering the same
> > power domain.
> > 
> > I think there are two possible optimizations there:
> > 
> > 1. use of a single memory barrier
> > 
> >    Since we are already em_pd_mutex protected, i.e. there cannot be a
> >    concurrent writers, we can use one single memory barrier after the
> >    loop, i.e.
> > 
> >         for_each_cpu(cpu, span)
> >                 WRITE_ONCE()
> >         smp_wmb()
> > 
> >    which should be just enough to ensure that all other CPUs will see
> >    the pointer set once we release the mutex
> 
> Right, I'm actually wondering if the memory barrier is needed at all ...
> The mutex lock()/unlock() should already ensure the ordering I want no ?
> 
> WRITE_ONCE() should prevent load/store tearing with concurrent em_cpu_get(),
> and the release/acquire semantics of mutex lock/unlock should be enough to
> serialize the memory accesses of concurrent em_register_perf_domain() calls
> properly ...
> 
> Hmm, let me read memory-barriers.txt again.

Yes, I think it should... but better double check.

> > 2. avoid using PER_CPU variables
> > 
> >    Apart from the initialization code, i.e. boot time, the em_data is
> >    expected to be read only, isn't it?
> 
> That's right. It's not only read only, it's also not read very often (in
> the use-cases I have in mind at least). The scheduler for example will
> call em_cpu_get() once when sched domains are built, and keep the
> reference instead of calling it again.
> 
> >    If that's the case, I think that using PER_CPU variables is not
> >    strictly required while it unnecessarily increases the cache pressure.
> > 
> >    In the worst case we can end up with one cache line for each CPU to
> >    host just an 8B pointer, instead of using that single cache line to host
> >    up to 8 pointers if we use just an array, i.e.
> > 
> >         struct em_perf_domain *em_data[NR_CPUS]
> >                 ____cacheline_aligned_in_smp __read_mostly;
> > 
> >    Consider also that: up to 8 pointers in a single cache line means
> >    also that single cache line can be enough to access the EM from all
> >    the CPUs of almost every modern mobile phone SoC.
> > 
> >    Note entirely sure if PER_CPU uses less overall memory in case you
> >    have much less CPUs then the compile time defined NR_CPUS.
> >    But still, if the above makes sense, you still have a 8x gain
> >    factor between number Write allocated .data..percp sections and
> >    the value of NR_CPUS. Meaning that in the worst case we allocate
> >    the same amount of memory using NR_CPUS=64 (the default on arm64)
> >    while running on an 8 CPUs system... but still we should get less
> >    cluster caches pressure at run-time with the array approach, 1
> >    cache line vs 4.
> 
> Right, using per_cpu() might cause to bring in cache things you don't
> really care about (other non-related per_cpu stuff), but that shouldn't
> waste memory I think. I mean, if my em_data var is the first in a cache
> line, the rest of the cache line will most likely be used by other
> per_cpu variables anyways ...
> 
> As you suggested, the alternative would be to have a simple array. I'm
> fine with this TBH. But I would probably allocate it dynamically using
> nr_cpu_ids instead of using a static NR_CPUS-wide thing though -- the
> registration of perf domains usually happens late enough in the boot
> process.
> 
> What do you think ?

Sound all reasonable to me.

> Thanks
> Quentin

Best Patrick

-- 
#include <best/regards.h>

Patrick Bellasi