linux-kernel - Re: [PATCH v3 1/1] doc: Add remote CPU access details and others to this_cpu

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Date:	Wed, 23 Jul 2014 19:05:54 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Randy Dunlap <rdunlap@...radead.org>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] doc: Add remote CPU access details and others to this_cpu_ops.txt

On Wed, Jul 23, 2014 at 6:58 PM, Christoph Lameter <cl@...two.org> wrote:
> On Sun, 20 Jul 2014, Pranith Kumar wrote:
>
>> -this_cpu ops are interrupt safe. Some architecture do not support
>> +this_cpu ops are interrupt safe. Some architectures do not support
>>  these per cpu local operations. In that case the operation must be
>>  replaced by code that disables interrupts, then does the operations
>>  that are guaranteed to be atomic and then reenable interrupts. Doing
>>  so is expensive. If there are other reasons why the scheduler cannot
>>  change the processor we are executing on then there is no reason to
>> -disable interrupts. For that purpose the __this_cpu operations are
>> -provided. For example.
>> +disable interrupts. For that purpose the following __this_cpu operations
>> +are provided.
>> +
>> +These operations have no guarantee against concurrent interrupts or
>> +preemption. If a per cpu variable is not used in an interrupt context
>> +and the scheduler cannot preempt then they are safe.
>> +
>> +Note that interrupts may still occur while an operation is
>> +in progress and if the interrupt too modifies the variable, then RMW
>> +actions may not be atomic.
>
> This paragraph is restating what the one before already said. Make these
> two one paragraph and condense a bit?

OK, I will fix this.

>
>
>> +Remote access to per cpu data
>> +------------------------------
>> +
>> +Per cpu data structures are designed to be used by one cpu exclusively.
>> +If you use the variables as intended, this_cpu_ops() are guaranteed to
>> +be "atomic" as no other CPU has access to these data structures. In this
>> +context, a remote access means an access to a per cpu data structure
>> +from a CPU without using the this_cpu_* operations.
>> +
>> +There are special cases where you might need to access per cpu data
>> +structures remotely. One such case is to perform maintenance tasks of
>> +idle CPUs without having to wake them up using IPIs for power saving
>> +purposes. It is usually safe to do a remote read access and that is
>> +frequently done to summarize counters. Remote write access is the one
>> +which is problematic. This is not recommended unless absolutely
>> +necessary. Remote write accesses to per cpu data structures are highly
>
> Two sentences making the same point.
>

The idea is to *strongly* discourage use of remote accesses ;)
I will fix this too.

>> +discouraged. Please consider using an IPI to wake up the remote CPU and
>> +perform the update to its per cpu area.
>
>
>> +
>> +
>> +     struct test {
>> +             atomic_t a;
>> +             int b;
>> +     };
>> +
>> +     DEFINE_PER_CPU(struct test, onecacheline);
>> +
>> +You cannot update the field 'a' remotely from one processor and expect
>> +this_cpu ops to work on the field b from the local CPU with atomic
>> +semantics. Care should be taken that such simultaneous access to data
>> +within the same cache line is avoided. Also costly synchronization is
>> +necessary when you are unsure of such cases. IPI is generally recommeded
>> +in such scenarios instead of remote write to per cpu areas.
>> +
>> +Given these drawbacks, it is adviced to seriously consider the option of
>> +not using per cpu areas when you have a high rate of remote writes.
>
> Well even any low late of remote writes still evicts the performance
> sensitive  cacheline from the processor that needs it. Should it be asleep
> and wake up then it will be missing the cacheline so the wakeup times are
> prolonged.
>
>

Yes, that can be the case. Should I add this para to the text?


-- 
Pranith
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