Re: [PATCH] memcg: remove unneeded preempt_disable

Johannes Weiner
2011-08-18T09:38:13Z

On Wed, Aug 17, 2011 at 11:50:53PM -0700, Greg Thelen wrote:
> Both mem_cgroup_charge_statistics() and mem_cgroup_move_account() were
> unnecessarily disabling preemption when adjusting per-cpu counters:
> preempt_disable()
> __this_cpu_xxx()
> __this_cpu_yyy()
> preempt_enable()
>
> This change does not disable preemption and thus CPU switch is possible
> within these routines. This does not cause a problem because the total
> of all cpu counters is summed when reporting stats. Now both
> mem_cgroup_charge_statistics() and mem_cgroup_move_account() look like:
> this_cpu_xxx()
> this_cpu_yyy()

Note that on non-x86, these operations themselves actually disable and
reenable preemption each time, so you trade a pair of add and sub on
x86

- preempt_disable()
__this_cpu_xxx()
__this_cpu_yyy()
- preempt_enable()

with

preempt_disable()
__this_cpu_xxx()
+ preempt_enable()
+ preempt_disable()
__this_cpu_yyy()
preempt_enable()

everywhere else.