Re: [Query] Ticks happen in pair for NO_HZ_FULL cores ?

Viresh Kumar
2013-12-18T04:38:22Z

On 17 December 2013 22:05, Kevin Hilman <khilman@linaro.org> wrote:
> For future reference, for generating email friendly trace output for
> discussion like this, you can use something like:
>
> trace-cmd report --cpu=1 trace.dat

Okay..

>> And after that the next event comes after 5 Seconds.
>>
>> And so I was talking for the Event 41.
>
> That first event (Event 41) is an interrupt, and comes from the
> scheduler tick. The tick is happening because the writeback workqueue
> just ran and we're not in NO_HZ mode.

This is what I was trying to ask. Why can't we enter in NO_HZ_FULL mode
as soon as writeback workqueue just ran? That way we can go into NOHZ
mode earlier..

> However, as soon as that IRQ (and resulting softirqs) are finished, we
> enter NO_HZ mode again. But as you mention, it only lasts for ~5 sec
> when the timer fires again. Once again, it fires because of the
> writeback workqueue, and soon therafter it switches back to NO_HZ mode
> again.

That's fine.. It wasn't part of my query :) .. But yes your trick
would be useful
for my usecase :)