Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hi Naveen,

On Thu, Apr 02, 2020 at 01:04:34PM +0530, Naveen N. Rao wrote:
[..snip..]

> >
> >It does reduce it to 10ms window. I am not sure if anyone samples PURR
> >etc faster than that rate.
> >
> >I measured how much time it takes to read the purr, spurr, idle_purr,
> >idle_spurr files back-to-back. It takes not more than 150us.  From
> >lparstat will these values be read back-to-back ? If so, we can reduce
> >the staleness_tolerance to something like 500us and still avoid extra
> >IPIs. If not, what is the maximum delay between the first sysfs file
> >read and the last sysfs file read ?
> 
> Oh, for lparstat usage, this is perfectly fine.
> 
> I meant that there could be other users of [s]purr who might care. I don't
> know of one, but since this is an existing sysfs interface, I wanted to
> point out that the behavior might change.

Fair point. Perhaps this should be documented in the Documentation, if
we are going to continue with this patch.

> 
> >
> >>
> >>I wonder if we should introduce a sysctl interface to control thresholding.
> >>It can default to 0, which disables thresholding so that the existing
> >>behavior continues. Applications (lparstat) can optionally set it to suit
> >>their use.
> >
> >We would be introducing 3 new sysfs interfaces that way instead of
> >two.
> >
> >/sys/devices/system/cpu/purr_spurr_staleness
> >/sys/devices/system/cpu/cpuX/idle_purr
> >/sys/devices/system/cpu/cpuX/idle_spurr
> >
> >I don't have a problem with this. Nathan, Michael, thoughts on this?
> >
> >
> >The alternative is to have a procfs interface, something like
> >/proc/powerpc/resource_util_stats
> >
> >which gives a listing similar to /proc/stat, i.e
> >
> >      CPUX  <purr>  <idle_purr>  <spurr>  <idle_spurr>
> >
> >Even in this case, the values can be obtained in one-shot with a
> >single IPI and be printed in the row corresponding to the CPU.
> 
> Right -- and that would be optimal requiring a single system call, at the
> cost of using a legacy interface.
> 
> The other option would be to drop this patch and to just go with patches 1-5
> introducing the new sysfs interfaces for idle_[s]purr. It isn't entirely
> clear how often this would be used, or its actual impact. We can perhaps
> consider this optimization if and when this causes problems...

I am ok with that. We can revisit the problem if IPI noise becomes
noticable. However, if Nathan or Michael feel that this problem is
better solved now, than leaving it for the future, we will have to
take a call on what the interface is going to be.

> 
> 
> Thanks,
> Naveen
> 

--
Thanks and Regards
gautham.