On 23/11/2018 17:20, Sudeep Holla wrote:
> On Fri, Nov 23, 2018 at 05:04:18PM +0100, Daniel Lezcano wrote:
>> On 23/11/2018 14:58, Sudeep Holla wrote:
>>> On Mon, Oct 29, 2018 at 05:23:18PM +0100, Daniel Lezcano wrote:
>>>> The mutex protects a per_cpu variable access. The potential race can
>>>> happen only when the cpufreq governor module is loaded and at the same
>>>> time the cpu capacity is changed in the sysfs.
>>>>
>>>
>>> I wonder if we really need that sysfs entry to be writable. For some
>>> reason, I had assumed it's read only, obviously it's not. I prefer to
>>> make it RO if there's no strong reason other than debug purposes.
>>
>> Are you suggesting to remove the READ_ONCE/WRITE_ONCE patch and set the
>> sysfs file read-only ?
>>
>
> Just to be sure, if we retain RW capability we still need to fix the
> race you are pointing out.
>
> However I just don't see the need for RW cpu_capacity sysfs and hence
> asking the reason here. IIRC I had pointed this out long back(not sure
> internally or externally) but seemed to have missed the version that got
> merged. So I am just asking if we really need write capability given that
> it has known issues.
>
> If user-space starts writing the value to influence the scheduler, then
> it makes it difficult for kernel to change the way it uses the
> cpu_capacity in future.
>
> Sorry if there's valid usecase and I am just making noise here.
It's ok [added Juri Lelli]
I've been through the history:
commit be8f185d8af4dbd450023a42a48c6faa8cbcdfe6
Author: Juri Lelli <juri.le...@arm.com>
Date: Thu Nov 3 05:40:18 2016 +0000
arm64: add sysfs cpu_capacity attribute
Add a sysfs cpu_capacity attribute with which it is possible to read and
write (thus over-writing default values) CPUs capacity. This might be
useful in situations where values needs changing after boot.
The new attribute shows up as:
/sys/devices/system/cpu/cpu*/cpu_capacity
Cc: Will Deacon <will.dea...@arm.com>
Cc: Mark Brown <broo...@kernel.org>
Cc: Sudeep Holla <sudeep.ho...@arm.com>
Signed-off-by: Juri Lelli <juri.le...@arm.com>
Signed-off-by: Catalin Marinas <catalin.mari...@arm.com>
Juri do you have a use case where we want to override the capacity?
Shall we switch the sysfs attribute to read-only?
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