Re: [PATCH 1/4] nohz: Only update sleeptime stats locally

[Stratos Karafotis]

On 08/18/2013 08:04 PM, Oleg Nesterov wrote:
> Sorry for double post. forgot to cc cpufreq maintainers.
> 
> On 08/16, Frederic Weisbecker wrote:
>>
>> To fix this, lets only update the sleeptime stats locally when the CPU
>> exits from idle.
> 
> I am in no position to ack the changes in this area, but I like this
> change very much. Because, as a code reader, I was totally confused by
> 
> 	if (last_update_time)
> 		update_ts_time_stats()
> 
> code and it looks "obviously wrong".
> 
> I added more cc's. It seems to me that 9366d840 "cpufreq: governors:
> Calculate iowait time only when necessary" doesn't realize what
> 
> 	-       u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
> 	+       u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);
> 
> actually means. OTOH, get_cpu_iowait_time_us() was called with
> last_update_time != NULL even before this patch...

To be honest, I am unfamiliar with tick-sched code.
With patch 9366d840, I was trying to avoid duplicate calls to
get_cpu_iowait_time_us function. I just saw that the original
code was calling update_ts_time_stats within get_cpu_idle_time_us
and get_cpu_iowait_time_us and I thought that I should keep calling
these functions with non NULL parameter to update the time stats.

In fact the original patch submission was without this:
-       u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
+       u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);
and the idle time calculation was wrong (ondemand couldn't increase to max freq)


For your convenience the call paths before and after this patch:

Before patch

get_cpu_idle_time(j, &cur_wall_time);
	u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
	idle_time += get_cpu_iowait_time_us(cpu, wall);
		update_ts_time_stats(cpu, ts, now, last_update_time);
... 
get_cpu_iowait_time_us(j, &cur_wall_time);
	update_ts_time_stats(cpu, ts, now, last_update_time);


After patch (io_busy = 1)

cur_idle_time = get_cpu_idle_time(j, &cur_wall_time, io_busy);
	u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);
		update_ts_time_stats(cpu, ts, now, last_update_time);


After patch (io_busy = 0)

cur_idle_time = get_cpu_idle_time(j, &cur_wall_time, io_busy);
	u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall : NULL);

	idle_time += get_cpu_iowait_time_us(cpu, wall);
		update_ts_time_stats(cpu, ts, now, last_update_time);


Regards,
Stratos
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