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

[Mathieu Desnoyers]

----- On Sep 16, 2016, at 3:58 PM, Mathieu Desnoyers mathieu.desnoyers at efficios.com wrote:

> ----- 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>
> 

Along with the following fixup:

commit a8e7c8d9eabfecc5017053754f6b446a95b00010
Author: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
Date:   Fri Sep 16 16:11:46 2016 -0400

    Fix: add missing CONFIG_RCU_FORCE_SYS_MEMBARRIER to urcu/config.h.in
    
    Expose this configuration define in the installed header.
    
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

Thanks,

Mathieu

> Thanks !!
> 
> Mathieu
> 
> 
>> 
>> Ciao, Duncan.
>> 
>>>
>>> Thanks,
>>>
>>> Mathieu
>>>
>>>>
>>>> Best wishes, Duncan.
>>>>
>>>> _______________________________________________
>>>> lttng-dev mailing list
>>>> lttng-dev at lists.lttng.org
>>>> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com