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

[Naveen N. Rao]

Gautham R Shenoy wrote:
> Hello Naveen,
>
>
> On Wed, Apr 01, 2020 at 03:28:48PM +0530, Naveen N. Rao wrote:
> > Gautham R. Shenoy wrote:
> > >From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>
> > >
>  [..snip..]
>
> > >+
> > >+static ssize_t show_purr(struct device *dev,
> > >+			 struct device_attribute *attr, char *buf)
> > > {
> > >-	u64 *ret = val;
> > >+	struct cpu *cpu = container_of(dev, struct cpu, dev);
> > >+	struct util_acct_stats *stats;
> > >
> > >-	*ret = read_this_idle_purr();
> > >+	stats = get_util_stats_ptr(cpu->dev.id);
> > >+	return sprintf(buf, "%llx\n", stats->latest_purr);
> >
> > This alters the behavior of the current sysfs purr file. I am not sure if it
> > is reasonable to return the same PURR value across a 10ms window.
>
>
> 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.

> > 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...


Thanks,
Naveen