Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Tim Chen]

On 5/12/21 6:59 AM, Qais Yousef wrote:
> On 05/11/21 10:25, Tim Chen wrote:
>>> update_next_balance() is only used in newidle_balance() so we could
>>> modify it to  have:
>>>
>>> next = max(jiffies+1, next = sd->last_balance + interval)
>>
>> Is the extra assignment "next = sd->last_balance + interval" needed?
>> This seems more straight forward:
>>
>> next = max(jiffies+1, sd->last_balance + interval)
> 
> I haven't been following the whole conversation closely, but it's always
> interesting when manipulating time in non time_*() functions.
> 
> Is this max() safe against wrapping?

Looking at the definition, seems like max doesn't take care of wrapping.
#define max(a, b) \
        ({ \
                typeof(a) __a = (a); \
                typeof(b) __b = (b); \
                MINMAX_ASSERT_COMPATIBLE(typeof(__a), typeof(__b)); \
                __a > __b ? __a : __b; \
        })


Probably need to do
next = time_after(jiffies+1, sd->last_balance + interval) ? jiffies+1 : sd->last_balance + interval;

Tim