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From: Nathan Lynch <>
To: Gautham R Shenoy <>
Cc: Tyrel Datwyler <>,,
	Kamalesh Babulal <>,
	Vaidyanathan Srinivasan <>,,
	"Naveen N. Rao" <>
Subject: Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr
Date: Fri, 03 Apr 2020 13:10:02 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Gautham R Shenoy <> writes:
> On Thu, Apr 02, 2020 at 01:04:34PM +0530, Naveen N. Rao wrote:
>> >>
>> >>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?

No, I don't think this warrants a tunable when the issue it's intended
to address is still a bit speculative at this point. (Also, note that
this would be a system-wide value, but you could have multiple
concurrent users of the interface with different needs.)

>> >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.

While I maintain some concern about the overhead on larger LPARs (150us
per CPU works out to ~0.15s total to serially sample 1024 CPUs, ~0.3s
for 2048 and so on), I am OK with the straightforward addition of the
attributes without any batching or sampling thresholds behind the scenes
for now. I appreciate your consideration of the issue.

If this turns out to be too inefficient then I think we should consider
a non-sysfs mechanism such as chardev+ioctl.

      reply	other threads:[~2020-04-03 18:10 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-27 11:32 [PATCH v4 0/6] [PATCH v4 0/6] Track and expose idle PURR and SPURR ticks Gautham R. Shenoy
2020-03-27 11:32 ` [PATCH v4 1/6] powerpc: Move idle_loop_prolog()/epilog() functions to header file Gautham R. Shenoy
2020-03-27 11:32 ` [PATCH v4 2/6] powerpc/idle: Add accessor function to always read latest idle PURR Gautham R. Shenoy
2020-04-01  9:42   ` Naveen N. Rao
2020-04-03  6:15     ` Gautham R Shenoy
2020-04-03 10:34       ` Naveen N. Rao
2020-04-03 11:24         ` Gautham R Shenoy
2020-03-27 11:32 ` [PATCH v4 3/6] powerpc/pseries: Account for SPURR ticks on idle CPUs Gautham R. Shenoy
2020-03-27 11:32 ` [PATCH v4 4/6] powerpc/sysfs: Show idle_purr and idle_spurr for every CPU Gautham R. Shenoy
2020-03-27 11:32 ` [PATCH v4 5/6] Documentation: Document sysfs interfaces purr, spurr, idle_purr, idle_spurr Gautham R. Shenoy
2020-04-01  9:45   ` Naveen N. Rao
2020-03-27 11:32 ` [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr Gautham R. Shenoy
2020-04-01  9:58   ` Naveen N. Rao
2020-04-01 12:01     ` Gautham R Shenoy
2020-04-02  7:34       ` Naveen N. Rao
2020-04-03  6:28         ` Gautham R Shenoy
2020-04-03 18:10           ` Nathan Lynch [this message]

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