Re: [PATCH v5 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp

Dietmar Eggemann
2020-11-12T13:05:59Z

On 11/11/2020 19:04, Peter Zijlstra wrote:
> On Wed, Nov 11, 2020 at 06:41:07PM +0100, Dietmar Eggemann wrote:
>> diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
>> index c852153ddb0d..b9165f17dddc 100644
>> --- a/include/uapi/linux/sched/types.h
>> +++ b/include/uapi/linux/sched/types.h
>> @@ -115,8 +115,8 @@ struct sched_attr {
>> __u64 sched_period;
>>
>> /* Utilization hints */
>> - __u32 sched_util_min;
>> - __u32 sched_util_max;
>> + __s32 sched_util_min;
>> + __s32 sched_util_max;
>
> So that's UAPI, not sure we can change the type here.

Yes, will remove this chunk. Not needed.

I probably should add some documentation there:

diff --git a/include/uapi/linux/sched/types.h
b/include/uapi/linux/sched/types.h
index c852153ddb0d..f2c4589d4dbf 100644
--- a/include/uapi/linux/sched/types.h
+++ b/include/uapi/linux/sched/types.h
@@ -96,6 +96,8 @@ struct sched_param {
* on a CPU with a capacity big enough to fit the specified value.
* A task with a max utilization value smaller than 1024 is more likely
* scheduled on a CPU with no more capacity than the specified value.
+ *
+ * A task utilization boundary can be reset by setting the attribute to -1.
*/
struct sched_attr {
__u32 size;

[...]

>> + if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
>> + util_max = attr->sched_util_max;
>> +
>> + if (util_max < -1 || util_max > SCHED_CAPACITY_SCALE)
>> + return -EINVAL;
>> + }
>
> Luckily we can write that range as a single branch like:
>
> if (util_{min,max} + 1 > SCHED_CAPACITY_SCALE+1)
>
> which assumes u32 :-)

Cool, will change it.

>> +
>> + if (util_min != -1 && util_max != -1 && util_min > util_max)
>> return -EINVAL;
>
> I think that will compile as is, otherwise write it like ~0u, which is
> the same bit pattern.

Yes, it compiles for me (arm64, gcc 9.2 and arm, gcc 8.3). Started a
0-Day build job to make sure.

Will do some more testing before sending out the updated version.