[lttng-dev] [PATCH] [URCU] Allow forcing the use of sys membarrier

[Mathieu Desnoyers]

----- On Sep 16, 2016, at 11:38 AM, Duncan Sands duncan.sands at deepbluecap.com wrote:

> Hi Mathieu,
> 
> On 09/12/2016 05:05 PM, Mathieu Desnoyers wrote:
>> ----- On Sep 6, 2016, at 9:07 AM, Duncan Sands duncan.sands at deepbluecap.com
>> wrote:
>>
>>> When using the RCU_MEMBARRIER flavour of userspace-RCU, kernel support for
>>> sys-membarrier is detected dynamically and stored in the rcu_has_sys_membarrier
>>> global variable.  Checking the value of this variable adds a small but
>>> measurable overhead to smp_mb_slave.  I only use userspace-rcu on systems which
>>> support sys-membarrier, and I'd like a way of avoiding that overhead.
>>>
>>> The attached patch makes the following changes: if RCU_FORCE_SYS_MEMBARRIER is
>>> defined then rcu_has_sys_membarrier is replaced with the constant 1, eliminating
>>> the overhead in smp_mb_slave.  As a sanity check, support for sys-membarrier is
>>> still detected at startup and if it isn't supported then the program aborts.
>>>
>>> I didn't try to integrate this feature into the build system (eg by adding a
>>> configure option for it) since I'm using my own build system.
>>
>> Hi Duncan,
>>
>> This is an interesting improvement. I'd be interested to merge it if it was
>> integrated with the liburcu build system. Perhaps a
>> --disable-dynamic-membarrier-check
>> at configure ?
> 
> how about the attached patch?

I pushed the following commit into urcu master, inspired from your patch,

commit d8d9a3405ce46af6d34d2e80e260ad50f3d211a0
Author: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
Date:   Fri Sep 16 15:15:41 2016 -0400

    Allow forcing the use of sys membarrier
    
    When using the default (liburcu.so) and bulletproof (liburcu-bp.so)
    flavours of Userspace RCU, kernel support for sys-membarrier is detected
    dynamically and stored in the rcu_has_sys_membarrier_memb and
    urcu_bp_has_sys_membarrier global variables.
    
    Checking the value of these variables adds a small but measurable overhead
    to smp_mb_slave. On systems which support sys-membarrier, it would be
    nice to have a way of avoiding that overhead.
    
    Here is the proposed approach: if CONFIG_RCU_FORCE_SYS_MEMBARRIER is
    defined then rcu_has_sys_membarrier_memb/urcu_bp_has_sys_membarrier are
    replaced with the constant 1, eliminating the overhead in smp_mb_slave.
    As a sanity check, support for sys-membarrier is still detected at
    startup. A program using liburcu or liburcu-bp compiled with this option
    aborts in the library constructor if the membarrier system call is not
    supported by the operating system.
    
    Suggested-by: Duncan Sands <duncan.sands at deepbluecap.com>
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

Thanks !!

Mathieu


> 
> Ciao, Duncan.
> 
>>
>> Thanks,
>>
>> Mathieu
>>
>>>
>>> Best wishes, Duncan.
>>>
>>> _______________________________________________
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com