Re: [RFC PATCH v1 1/6] sched: track time spent in IRQ handler

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Hi Dario,

On Fri, 2020-06-12 at 22:08 +0200, Dario Faggioli wrote:
> On Fri, 2020-06-12 at 13:21 +0100, Julien Grall wrote:
> > On 12/06/2020 12:33, Volodymyr Babchuk wrote:
> > > On Fri, 2020-06-12 at 12:29 +0100, Julien Grall wrote:
> > > > > Basically, this value holds time span between calls to
> > > > > schedule(). This
> > > > > variable gets zeroed out every time scheduler requests for time
> > > > > adjustment value. So, it should not depend on total VM run
> > > > > time.
> > > > This is assuming that the scheduler will be called. With the NULL
> > > > scheduler in place, there is a fair chance this may never be
> > > > called.
> > > > 
> Yeah, this is a good point. I mean, I wouldn't be sure about "never",
> as even there, we'd probably have softirqs, tasklets, etc... And I
> still have to look at these patches in more details to figure out
> properly whether they'd help for this.

Well. I think, it is possible to reset counters when we are switching
to a different scheduler. Just for cases like that.

> But I'd say that, in general, we should depend of the frequency of the
> scheduling events as few as possible. Therefore, using 64 bits from the
> start would be preferrable IMO.

I should done that calculation earlier... So, it appears that 32 bit
counter can count up to 4 mere seconds. It should be enought for normal
flow. But I'm agree with you - 64 bits looks much safer. 

> 
> > > > So I think using a 64-bit value is likely safer.
> Yep.
> 
> > > Well, I wanted to use 64-bit value in the first place. But I got
> > > the
> > > impression that atomic_t supports only 32-bit values. At least,
> > > this is
> > > what I'm seeing in atomic.h
> > > 
> > > Am I wrong?
> > 
> > There is no atomic64_t support in Xen yet. It shouldn't be very 
> > difficult to add support for it if you require them.
> > 
> Cool! That would be much appreciated. :-D
> 

Certainly! :)

I believe, there will be another users for atmic64_t as well.