linux-kernel - Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle

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Message-Id: <1585811157.uig8s95yst.naveen@linux.ibm.com>
Date:   Thu, 02 Apr 2020 13:04:34 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     ego@...ux.vnet.ibm.com, Michael Ellerman <mpe@...erman.id.au>
Cc:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading
 [idle_][s]purr

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@...ux.vnet.ibm.com>
>> >
>  [..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